Cooked a fish on style (7)
I believe the answer is:
fashion
'style' is the definition.
(I know that style can be written as fashion)
'cooked a fish on' is the wordplay.
'cooked' indicates anagramming the letters (letters cooked into a new form).
'a'+'fish'='afish'
'afish' with letters rearranged gives 'fashi'.
'fashi'+'on'='FASHION'
(Other definitions for fashion that I've seen before include "Vogue, style" , "Ton" , "Haute couture" , "way" , "Make; popular trend" .)