Plain informant to alight (9)
I believe the answer is:
grassland
'plain' is the definition.
(I've seen this before)
'informant to alight' is the wordplay.
'informant' becomes 'grass' (term for an informant or traitor).
'to alight' becomes 'land' (I've seen this before).
'grass'+'land'='GRASSLAND'
(Other definitions for grassland that I've seen before include "green countryside" , "Meadow or prairie" , "pasture" , "Plain" , "(growth area)" .)