Spray operation set up against society's outsiders (4)
I believe the answer is:
posy
'spray' is the definition.
(I've seen this before)
'operation set up against society's outsiders' is the wordplay.
'operation' becomes 'op'.
'set up' is a reversal indicator (in a down clue, letters go up).
'against' means one lot of letters go next to another.
'outsiders' says to hollow out the word (remove centre letters) (only the letters on the outside of the word remain).
'society' with its centre removed is 'sy'.
'op' backwards is 'po'.
'po'+'sy'='POSY'
(Other definitions for posy that I've seen before include "Small bunch of flowers" , "'Corsage, nosegay (4)'" , "Pretentious-sounding" , "Spray" , "Small bouquet" .)