Fish with twit on banks (6)
I believe the answer is:
wrasse
'fish' is the definition.
(wrasse is a kind of fish)
'with twit on banks' is the wordplay.
'with' becomes 'w' (abbreviation).
'twit on banks' becomes 'rasse' (I can't justify this - if you can you should believe this answer much more).
'w'+'rasse'='WRASSE'
(Other definitions for wrasse that I've seen before include "Type of fish" , "Brightly-coloured fish" , "catch in the deep" , "Swimmer" , "Swears about the tropical marine fish with strong teeth" .)