Unstable man skirting grass (7)
I believe the answer is:
erratic
'unstable' is the definition.
(I've seen this in another clue)
'man skirting grass' is the wordplay.
'man' becomes 'eric' (I've seen this before).
'skirting' indicates putting letters inside.
'grass' becomes 'rat' (both can mean a snitch or informer).
'eric' placed around 'rat' is 'ERRATIC'.
(Other definitions for erratic that I've seen before include "Random, unreliable" , "Fluctuating" , "Inconsistent, shifting" , "Fickle" , "Changeable" .)