One unsuccessful in different roles (5)
I believe the answer is:
loser
'one unsuccessful' is the definition.
(I've seen this before)
'different roles' is the wordplay.
'different' indicates an anagram.
'roles' anagrammed gives 'LOSER'.
'in' is the link.
(Other definitions for loser that I've seen before include "One who is (always) unsuccessful" , "One vanquished" , "Failed competitor" , "Defeated person" , "Roles for the defeated" .)