Upset by rotten conclusion (6)
I believe the answer is:
offend
'upset' is the definition.
(offend can mean to upset or insult)
'by rotten conclusion' is the wordplay.
'by' 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'+'end' is 'OFFEND'.
(Other definitions for offend that I've seen before include "Be disagreeable to" , "Outrage" , "Cause displeasure to, bruise feelings" , "Wound (one's) feelings" , "Upset; transgress" .)