It's a fish? Smart boy! (8)
I believe the answer is:
stingray
'it's a fish?' is the definition.
(stingray is a kind of fish)
'smart boy' is the wordplay.
'smart' becomes 'sting' (stinging is a kind of smarting).
'boy' becomes 'ray' (**).
'sting'+'ray'='STINGRAY'
(Other definitions for stingray that I've seen before include "menace in the water" , "Flatfish with barbed spine" , "swimmer" , "Large fish with tail spine" , "Dangerous fish" .)