Fertile places - rat arrives (10)

I believe the answer is:

grasslands

'fertile places' is the definition.

'rat arrives' is the wordplay.
'rat' becomes 'grass' (slang term for an informer).
'arrives' becomes 'lands' (landing is a kind of arriving).
'grass'+'lands'='GRASSLANDS'

(Another definition for grasslands that I've seen is " The pampas, for example".)

I've seen this clue in The Telegraph.

Want a hint initially instead of a full solution? Install my app