Crossword setter's not quite stupid or fabulous (6)
I believe the answer is:
mythic
'fabulous' is the definition.
(synonyms)
'crossword setter's not quite stupid' is the wordplay.
'crossword setter's' becomes 'my' (belonging to the person writing the clues).
'not quite' means to remove the last letter.
'stupid' becomes 'thick' (similar in meaning).
'thick' with its last letter taken away is 'thic'.
'my'+'thic'='MYTHIC'
'or' is the link.
(Other definitions for mythic that I've seen before include "legendary" , "Fabulous, fictitious" .)