Runner-up to exchange roles (5)
I believe the answer is:
loser
'runner-up' is the definition.
(runner-up can mean anyone other than the winner)
'exchange roles' is the wordplay.
'exchange' is an anagram indicator.
'roles' with letters rearranged gives 'LOSER'.
'to' acts as a link.
(Other definitions for loser that I've seen before include "One failing regularly" , "Person who's a failure" , "lemon" , "One who frequently fails (colloq.)" , "Winner's opponent" .)