One defeated when swapping roles (5)
I believe the answer is:
loser
'one defeated' is the definition.
'when swapping roles' is the wordplay.
'when swapping' indicates anagramming the letters (letters swap places with each other).
'roles' with letters rearranged gives 'LOSER'.
(Other definitions for loser that I've seen before include "Flop" , "Roles for a non-winner" , "No-hoper" , "One who regularly fails" , "Proverbially, one who hesitates" .)