Top crime to upset rival (10)
I believe the answer is:
competitor
'rival' is the definition.
(I know that rival can be written as competitor)
'top crime to upset' is the wordplay.
'upset' indicates anagramming the letters.
'top'+'crime'+'to'='topcrimeto'
'topcrimeto' with letters rearranged gives 'COMPETITOR'.
(Other definitions for competitor that I've seen before include "Sporting contest entrant" , "rival trader?" .)