I'll be upset after start of conflict, since America is what produces it (5,5)
I believe the answer is:
casus belli
'what produces it' is the definition.
I can't tell whether this defines the answer.
'i'll be upset after start of conflict since america' is the wordplay.
'upset' indicates an anagram.
'after' is a charade indicator (letters next to each other).
'start of' indicates taking the first letters.
'since' becomes 'as' (both can mean 'because').
'america' becomes 'us' (United States).
The initial letter of 'conflict' is 'c'.
'ill'+'be'='illbe'
'illbe' with letters rearranged gives 'belli'.
'c'+'as'+'us'='casus'
'belli' put after 'casus' is 'CASUS BELLI'.
'is' acts as a link.
Can you help me to learn more?
(Other definitions for casus belli that I've seen before include "reason for aggression" , "Origin of conflict" , "Source of conflict" , "Justification for going to war (Latin)" .)