Listened to cat, shark? (7)
I believe the answer is:
cheater
'shark?' is the definition.
'listened to cat' is the wordplay.
'listened to' shows a homophone (sound like).
'cat' becomes 'cheetah' (cheetah is a kind of cat).
'cheetah' sounds like 'CHEATER'.
(Other definitions for cheater that I've seen before include "Dishonest person" , "Person winning unfairly" , "Rook" , "The race (anag)" , "Person flouting rules" .)