Informer comprehends losing power (5)
I believe the answer is:
grass
'informer' is the definition.
(grass can mean an informant)
'comprehends losing power' is the wordplay.
'comprehends' becomes 'grasps' ('grasp' can be a synonym of 'comprehend').
'losing' suggests deleting specific letters.
'power' becomes 'p'.
'grasps' with 'p' removed is 'GRASS'.
(Other definitions for grass that I've seen before include "Dope" , "Pasture" , "Informer, nark" , "copper may rely on him?" , "often walked over in court" .)