A representative leaving mushroom and bun (7)
I believe the answer is:
'bun' is the definition.
I cannot understand how the rest of the clue works.
(Other definitions for chignon that I've seen before include "Arrangement of coil of hair" , "Roll of hair worn at the nape" , "Roll of hair worn at the back of the head" , "Hairstyle" , "Topknot" .)