Indeed upset when frustrated (6)
I believe the answer is:
denied
'frustrated' is the definition.
Although both the answer and definition are past participle verbs, I don't see how they can define each other.
'indeed upset' is the wordplay.
'upset' is an anagram indicator.
'indeed' with letters rearranged gives 'DENIED'.
'when' is the link.
Can you help me to learn more?
(Other definitions for denied that I've seen before include "Refused to admit and refused access to" , "Contradicted" , "Indeed (anag)" , "Declared something to be untrue" , "Refused to give (someone something)" .)