Reason one upset with a trial (9)
I believe the answer is:
rationale
'reason' is the definition.
(I've seen this before)
'one upset with a trial' is the wordplay.
'upset with' indicates an anagram.
'a'+'trial'='atrial'
'one' anagrammed with 'atrial' gives 'RATIONALE'.
(Other definitions for rationale that I've seen before include "justification" , "Logical reasons (for action)" , "This might account for" , "Philosophy" , "Good reason" .)