Accomplice holding ecstasy in backstreet (5)
I believe the answer is:
alley
'backstreet' is the definition.
(I've seen this in another clue)
'accomplice holding ecstasy' is the wordplay.
'accomplice' becomes 'ally' (both can mean a colleague or associate).
'holding' means one lot of letters goes inside another.
'ecstasy' becomes 'E' (informal term for the drug).
'ally' placed around 'e' is 'ALLEY'.
'in' is the link.
(Other definitions for alley that I've seen before include "Passage between houses, perhaps" , "Narrow back street" , "Walk" , "Narrow lane; type of cat" , "Narrow passage" .)