Marijuana for the informer (5)
I believe the answer is:
grass
This clue is a double definition.
'marijuana' is the first definition.
(I know that grass is a type of marijuana)
'the informer' is the second definition.
(grass can mean an informant)
'for' acts as a link.
(Other definitions for grass that I've seen before include "Drug" , "Soporific" , "Plant used for lawns" , "Weed" , "sing?" .)