Vegetarian food can upset a lad! (3)
I believe the answer is:
dal
'vegetarian food' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'upset a lad' is the wordplay.
'upset' shows that the letters should be reversed in order.
'lad' in reverse letter order is 'DAL'.
'can' is the link.
Can you help me to learn more?
(Other definitions for dal that I've seen before include "Pigeon pea" , "Indian dish of pulses" , "Indian pulse puree" .)