Legendary setter's not completely stupid (6)
I believe the answer is:
mythic
'legendary' is the definition.
(I know that legendary can be written as mythic)
'setter's not completely stupid' is the wordplay.
'setter's' becomes 'my' (belonging to the crossword setter).
'not completely' means to remove the last letter (the word is incomplete).
'stupid' becomes 'thick' (similar in meaning).
'thick' with its final letter taken away is 'thic'.
'my'+'thic'='MYTHIC'
(Other definitions for mythic that I've seen before include "Fabulous, fictitious" , "legendary" .)