When upset, I go off doughnuts (4)
I believe the answer is:
tori
'doughnuts' is the definition.
(I've seen this before)
'when upset i go off' is the wordplay.
'when upset' says the letters should be written in reverse (in a down clue - set, written, up).
'go off' becomes 'rot' (as in food going off).
'i'+'rot'='irot'
'irot' in reverse letter order is 'TORI'.
(Other definitions for tori that I've seen before include "Large mouldings on columns" , "rings" , "doughnuts" .)