Insult by sending cakes back (4)
I believe the answer is:
snub
'insult by sending' is the definition.
'snub' can be an answer for 'insult' (snub can mean to insult or humiliate). I'm not sure about the 'by sending' bit.
'cakes back' is the wordplay.
'cakes' becomes 'buns' (cake is a type of bun).
'back' says the letters should be written backwards.
'buns' written backwards gives 'SNUB'.
Can you help me to learn more?
(Other definitions for snub that I've seen before include "Rudely ignore" , "Insult by ignoring" , "put down" , "Treat with disdain" , "Rebuff; brush-off" .)