Informant on lawn (5)
I believe the answer is:
grass
This clue is a double definition.
'informant' is the first definition.
(term for an informant or traitor)
'lawn' is the second definition.
(I've seen this before)
'on' acts as a link.
(Other definitions for grass that I've seen before include "Perhaps bent" , "sing?" , "Turf" , "what may be underfoot?" , "Court's covering" .)