A great ape In Goa stream (7)
I believe the answer is:
'a great ape' is the definition.
(gorilla is a kind of great ape)
'in goa stream' is the wordplay.
'in' is an insertion indicator.
'stream' becomes 'rill' (rill is a kind of stream).
'goa' going around 'rill' is 'GORILLA'.
(Other definitions for gorilla that I've seen before include "A large powerful African ape" , "Large animal" , "Heavy" , "Endangered anthropoid" , "Big ape" .)