Fantastic fish ain't cooked (9)
I believe the answer is:
brilliant
'fantastic' is the definition.
'fish ain't cooked' is the wordplay.
'fish' becomes 'brill' (brill is a kind of fish).
'cooked' is an anagram indicator (letters cooked into a new form).
'aint' anagrammed gives 'iant'.
'brill'+'iant'='BRILLIANT'
(Other definitions for brilliant that I've seen before include "Vivid; clever" , "Very bright; marvellous" , "Shining intensly" , "Exceptionally clever" , "Glittering" .)