Swimmer swears violently (6)
I believe the answer is:
wrasse
'swimmer' is the definition.
(wrasse is a kind of aquatic vertebrate)
'swears violently' is the wordplay.
'violently' indicates anagramming the letters.
'swears' with letters rearranged gives 'WRASSE'.
(Other definitions for wrasse that I've seen before include "Bony fish" , "catch in the deep" , "Swimmer" , "Brightly-coloured fish" , "Type of fish" .)