Discovered I end diet if upset (10)
I believe the answer is:
identified
'discovered' is the definition.
('identify' can be a synonym of 'discover')
'i end diet if upset' is the wordplay.
'upset' is an anagram indicator.
'end'+'diet'+'if'='enddietif'
'enddietif' anagrammed gives 'dentified'.
'i'+'dentified'='IDENTIFIED'
(Other definitions for identified that I've seen before include "Recognized" , "Recognised, named" , "Pointed out; made known" , "Given name" , "Put one's finger on" .)