Not even upset at the bitter end (5)
I believe the answer is:
never
'not' is the definition.
(thesaurus)
'even upset at the bitter end' is the wordplay.
'upset' is a reversal indicator.
'at' means one lot of letters go next to another.
'end' suggests the final letters.
The final letter of 'bitter' is 'r'.
'even' backwards is 'neve'.
'neve'+'r'='NEVER'
(Other definitions for never that I've seen before include "Not in a million years!" , "On no account" , "I refuse" , "' - - Say - - Again': Bond film" , "At no time in the past or future" .)