Sending the food back is an insult (4)
I believe the answer is:
snub
'an insult' is the definition.
(snub can mean to insult or humiliate)
'sending the food back' is the wordplay.
'sending' becomes 's' (this could be a standard abbreviation I've not previously seen).
'the food' becomes 'bun' (bun is a kind of food).
'back' is a reversal indicator.
'bun' back-to-front is 'nub'.
's'+'nub'='SNUB'
'is' is the link.
Can you help me to learn more?
(Other definitions for snub that I've seen before include "Rude rejection" , "Rude put-down; blunt (nose)" , "put down" , "Rebuff with buns" , "Shun" .)