He is upset by denial (6)
I believe the answer is:
daniel
'he' is the definition.
'upset by denial' is the wordplay.
'upset by' indicates an anagram.
'denial' anagrammed gives 'DANIEL'.
'is' is the link.
(Other definitions for daniel that I've seen before include "Biblical prophet who survived the lion's den" , "-- Defoe; -- Barenboim" , "- Defoe, English writer" , "Lions' den survivor" , "A wise man come to judgement (M of V)" .)