Supporter of the arts decapitating nasty piece of work (5)
I believe the answer is:
easel
'supporter of the arts' is the definition.
(supporter for an artist's canvas)
'decapitating nasty piece of work' is the wordplay.
'decapitating' means to remove the first letter.
'nasty piece of work' becomes 'weasel' (weasel can mean a nasty person).
'weasel' with its initial letter taken off is 'EASEL'.
(Other definitions for easel that I've seen before include "board support" , "artist's requisite" , "Artist's supporting frame" , "Painting support" , "Stand used by artists" .)