Fish ain't terribly bright (9)
I believe the answer is:
brilliant
'bright' is the definition.
(synonyms)
'fish ain't terribly' is the wordplay.
'fish' becomes 'brill' (brill is a kind of fish).
'terribly' indicates an anagram.
'aint' anagrammed gives 'iant'.
'brill'+'iant'='BRILLIANT'
(Other definitions for brilliant that I've seen before include "Shining intensly" , "Very bright or vivid" , "Bright and sparkling" , "Of surpassing excellence" , "'Wonderful, dazzling (9)'" .)