Spurn buns that are returned (4)
I believe the answer is:
snub
'spurn' is the definition.
(snubbing is a kind of spurning)
'buns that are returned' is the wordplay.
'that are returned' says the letters should be written in reverse.
'buns' written backwards gives 'SNUB'.
(Other definitions for snub that I've seen before include "Reject or humiliate person" , "Blank" , "Rude put-down; blunt (nose)" , "Ignore, send to Coventry" , "Rebuff, spurn" .)