Grass inside crate (3)
I believe the answer is:
rat
'grass' is the definition.
(both can mean a snitch or informer)
'inside crate' is the wordplay.
'inside' says the answer is hidden in the clue.
'RAT' can be found hidden inside 'crate'.
(Other definitions for rat that I've seen before include "Renegade" , "Betrayer" , "Desert - rodent" , "Tell-tale" , "Sewer rodent" .)