Nice mushroom a representative has left for bun (7)
I believe the answer is:
chignon
'bun' is the definition.
(thesaurus)
I don't understand how the remainder of the clue works.
(Other definitions for chignon that I've seen before include "Topknot" , "Coil of hair at the back of the head" , "Hairstyle" , "Roll of hair worn at the nape" , "Roll of hair worn at the back of the head" .)