Three men inject drug for benefit (4)
I believe the answer is:
perk
'benefit' is the definition.
(perk is a kind of benefit)
'three men inject drug' is the wordplay.
'three men' can mean three substitutions for 'man'.
'man' becomes 'p' (abbreviation for pawn, a chess piece or 'man').
'man' becomes 'r' (abbreviation for rook, a chess piece or 'man').
'man' becomes 'k' (abbreviation for king, a chess man).
'inject' is an insertion indicator.
'drug' becomes 'e' (e is another name for ecstasy).
'p'+'r'+'k'='prk'
'prk' placed around 'e' is 'PERK'.
'for' is the link.
(Other definitions for perk that I've seen before include "Bonus" , "Employee's benefit as an entitlement of job" , "Informal goodie given on top of salary" , "Tip" , "Company car, for example" .)