Upset with rotten conclusion (6)
I believe the answer is:
offend
'upset' is the definition.
(offend can mean to upset or insult)
'with rotten conclusion' is the wordplay.
'with' means one lot of letters go next to another.
'rotten' becomes 'off' (rotten food is 'off').
'conclusion' becomes 'end' (something's conclusion is its end).
'off' next to 'end' is 'OFFEND'.
(Other definitions for offend that I've seen before include "Cause affront or do wrong" , "Hurt someone's feelings" , "Aggrieve" , "Act in disregard of the law" , "Upset - break the law" .)