Informant (5)
I believe the answer is:
grass
'informant' is the definition.
(term for an informant or traitor)
This is all the clue.
(Other definitions for grass that I've seen before include "Shopper" , "What all flesh is (Isaiah; 1 Peter)" , "copper may rely on him?" , "Narrow-leaved green herbage" , "Pasturage" .)