Excellent fish ain't at sea! (9)
I believe the answer is:
brilliant
'excellent' is the definition.
(thesaurus)
'fish ain't at sea' is the wordplay.
'fish' becomes 'brill' (brill is a kind of fish).
'at sea' is an anagram indicator.
'aint' anagrammed gives 'iant'.
'brill'+'iant'='BRILLIANT'
(Other definitions for brilliant that I've seen before include "Brainy" , "Exceptionally clever" , "Superb" , "Lustrous" , "'Wonderful, dazzling (9)'" .)