ACROSS
1 Flyer leaves copilot down cushion (6)
PILLOW - cushion (def) from PIL (copilot without coot, a bird (flyer)) LOW (down) 

4 Peculiar dances for eccentrics (8)
ODDBALLS - eccentrics (def) from ODD (peculiar) BALLS (dances)

9 Leaders of weight-loss initiative waste away inside (6)
WITHIN - inside ( def) from WI (First letters (Leaders) of Weight-loss Initiative) THIN (waste away)

10 Not compulsory for one to join deployed platoon (8)
OPTIONAL - Not compulsory (def) from anagram (deployed) of I (one) and PLATOON
 
11 Make exceptional effort to leave home of Owen, say, demolished (2,3,2,4,3)
GO OUT OF ONE'S WAY - Make exceptional effort (def) from GO OUT (leave home) OF with anagram (demolished) of  OWEN SAY

13 Sun eg, with another often seen as one (6,4)
BINARY STAR - Cryptic def STAR (sun eg) with another often seen as one - Binary stars are systems of two stars orbiting a common centre that often appear as a single star when viewed through telescopes

14 To some, dreadful sounding colourist (4)
DYER - colourist (def) homophone to some listeners of DIRE (dreadful sounding)

16 Twist around web address (4)
CURL - Twist (def) from C (around) URL (web address) 

18 Productively produce completely (10)
FRUITFULLY - Productively (def) from FRUIT (produce) FULLY (completely)

21 Lucky for some when foul farce is disrupted by admirer (4-4,6)
FOUR-LEAF CLOVER - Lucky for some (def) - some people think four-leaf clovers bring good luck from anagram (is disrupted) of FOUL FARCE with LOVER (admirer)

23 Empty talk Cher trio exchanged (8)
RHETORIC - Empty talk (def) from anagram (exchanged) of CHER TRIO

24 The man follows court order to squirm (6)
WRITHE - squirm (def) from WRIT (court order) HE (the man follows) 

25 Recognises partners behind recording with royal (8)
DISCERNS - Recognises (def) from NS (partners - North and South in Bridge) behind DISC (recording) with ER (royal)

26 Ragged little Edward holds back postage-paid letter to himself (6)
TEASED - Ragged (def) from TED (little Edward) holding SEA (SAE reversed)

DOWN
1 Piece to give as security (4)
PAWN - Piece (def - chess piece) to give as security (def) 

2 Allow director admit disappointment (3-4)
LET-DOWN - disappointment (def) from LET (allow) D (director) OWN (admit)
 
3 Late announcement? (8)
OBITUARY - Cryptic def

5 One overly fond of the drink is very obsessive person after swim (10)
DIPSOMANIAC - One overly fond of the drink (def) from SO (very) MANIAC (obsessive person) after DIP (swim)

6 Horse around French island court (6)
BAILEY - court (def) from BAY (horse) containing (around) ILE (French island)

7 Again found in song passage (7)
LANEWAY - passage (def) from ANEW (again) in LAY (song)

8 Sudden rush to host charity informally (5,4)
SALLY ARMY - charity informally (def) from SALLY (sudden rush) to ARMY (host)

12 Vibrate so strangely dealing with remark (11)
OBSERVATION - remark (def) from anagram (strangely) of VIBRATE SO and ON (dealing with)

13 Had opposite effect when support was passionately inflamed (9)
BACKFIRED - Had opposite effect (def) from BACK (support) FIRED (was passionately inflamed)

15 Certainly straying less strongly (2,6)
OF COURSE - Certainly (def) from OFF COURSE (straying) less F (loudly)

17 Tools news agency swaps over for energy (7)
ROUTERS - Tools (def) from REUTERS (news agency) with O replacing E (swaps over for energy)

19 Real turnaround and setback for poisoned sword wielder (7)
LAERTES - poisoned sword wielder (def - Laertes kills Hamlet with a poisoned sword)  from LAER (REAL turned arond) and TES (SET back)

20 Pub landlord's first moulding in recess (6)
ALCOVE - recess (def) from AL (Pub landlord's (Al Murray) first) COVE (moulding)
 
22 Elected Irish politician keeps space for nurse (4)
TEND - nurse (def) from TD (Elected Irish politician) keeping EN (space - a typographic unit)