Foolish boss accepting Greek letter (6)
I believe the answer is:
stupid
'foolish' is the definition.
(synonyms)
'boss accepting greek letter' is the wordplay.
'boss' becomes 'stud' (both can mean a metal decoration).
'accepting' is an insertion indicator (inserted letters are accepted).
'greek letter' becomes 'pi' (letter from the Greek alphabet).
'stud' placed around 'pi' is 'STUPID'.
(Other definitions for stupid that I've seen before include "Brainless" , "Asinine" , "Ass" , "Daft" , "Dull" .)