Returning the cakes would be an insult (4)
I believe the answer is:
Here is my best explanation:
'insult' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
I cannot understand how the remainder of the clue works.
(Other definitions for buns that I've seen before include "Food items", "Cakes" and "Small rolls".)