He doesn't keep changing roles (5)
I believe the answer is:
loser
'he doesn't keep' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'changing roles' is the wordplay.
'changing' indicates an anagram.
'roles' anagrammed gives 'LOSER'.
Can you help me to learn more?
(Other definitions for loser that I've seen before include "One not winning" , "A failure, also-ran" , "Washout" , "Flop" , "Person who's a failure" .)