Leave stream to a large animal (7)
I believe the answer is:
gorilla
'large animal' is the definition.
(I've seen this before)
'leave stream to a' is the wordplay.
'leave' becomes 'go' (to go is to depart or leave).
'stream' becomes 'rill' (rill is a kind of stream).
'go'+'rill'+'a'='GORILLA'
(Other definitions for gorilla that I've seen before include "big mammal" , "huge beast" , "A large powerful African ape" , "Endangered anthropoid" , "brutal man" .)