Daniel upset: it's rejection (6)
I believe the answer is:
denial
'it's rejection' is the definition.
(rejection of a claim)
'daniel upset' is the wordplay.
'upset' indicates anagramming the letters.
'daniel' is an anagram of 'DENIAL'.
(Other definitions for denial that I've seen before include "Saying it isn't so" , "Refusal to acknowledge" , "Refusal to believe an unacceptable truth" , "Declaration that something is untrue" , "Refusal to admit painful truths" .)