Top model’s rear (5)
I believe the answer is:
trump
'top' is the definition.
(both can mean to beat or outdo)
'model's rear' is the wordplay.
'model' becomes 'T' (Ford Model T car).
'rear' becomes 'rump' (synonyms).
't'+'rump'='TRUMP'
(Other definitions for trump that I've seen before include "reliable person" , "A playing card given added value in some games" , "High-ranking card" , "Card of the most powerful suit" , "Card of designated top suit" .)