It can hold a bun or a buck, you might say (7)
I believe the answer is:
hairnet
'it can hold a bun' is the definition.
(bun is a type of hairstyle)
I don't understand how the remainder of the clue works.
(Other definitions for hairnet that I've seen before include "It protects locks" , "It keeps locks in place" , "protection overhead" , "Cover for head" , "In heart (anag)" .)