Not completely genuine politician to win surprising victory? (5)
I believe the answer is:
trump
'to win surprising victory?' is the definition.
(to trump is to outdo or upstage)
'not completely genuine politician' is the wordplay.
'not completely' means to remove the last letter.
'genuine' becomes 'true' (similar in meaning).
'politician' becomes 'mp' (Member of Parliament).
'true' with its last letter removed is 'tru'.
'tru'+'mp'='TRUMP'
(Other definitions for trump that I've seen before include "reliable person" , "Surpass" , "This card has an advantage" , "Playing card with designated added value" , "Valuable card" .)