Informer to come down to open country (9)
I believe the answer is:
grassland
'open country' is the definition.
(thesaurus)
'informer to come' is the wordplay.
'informer' becomes 'grass' (grass can mean an informant).
'to' means one lot of letters go next to another.
'come' becomes 'land' (landing is a kind of coming).
'grass'+'land'='GRASSLAND'
'down to' is the link.
(Other definitions for grassland that I've seen before include "Meadow or prairie" , "(growth area)" , "Large area suitable for grazing" , "Plain" , "pasture" .)