Set aside a returned Bath bun Sally’s not wanted (5)
I believe the answer is:
annul
'set aside' is the definition.
(to annul is to cancel or set aside)
'a returned bath bun sally's not wanted' is the wordplay.
I cannot quite see how this works, but
'a' could be 'an' and 'an' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for annul that I've seen before include "Cancel, make void" , "Cancel or render void" , "scratch" , "Quash" , "Nullify" .)