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