Marijuana shop? (5)
I believe the answer is:
grass
This clue is a double definition.
'marijuana' is the first definition.
('grass' can be a synonym of 'marijuana')
'shop?' is the second definition.
(both are slang for 'to inform on')
(Other definitions for grass that I've seen before include "copper may rely on him?" , "German writer" , "that often gets cut" , "Vegetation; informer" , "Pasture" .)