Churchill is victorious — then not, after upset (7)
I believe the answer is:
winston
'churchill' is the definition.
(UK prime minister Winston Churchill)
'is victorious then not after upset' is the wordplay.
'is victorious' becomes 'wins' (to win is to be victorious).
'then' says to put letters next to each other.
'after upset' is a reversal indicator (in a down clue, letters go up).
'not' backwards is 'ton'.
'wins'+'ton'='WINSTON'
(Other definitions for winston that I've seen before include "Statesman" , "Prime minister" , "war leader" , "-- Churchill, UK PM" , "11 [LAWMAKER]" .)