Toy with some fish, say? It's not a fish (7)
I believe the answer is:
dolphin
'it's not a fish' is the definition.
(dolphins are mammals)
I don't understand how the rest of the clue works.
(Other definitions for dolphin that I've seen before include "swimmer" , "Relative of whale" , "Small (bottlenosed) whale" , "Intelligent marine mammal" , "Small sociable whale" .)