One who cannot keep switching roles (5)
I believe the answer is:
loser
'one who cannot keep' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'switching roles' is the wordplay.
'switching' indicates an anagram (letters switch places).
'roles' anagrammed gives 'LOSER'.
Can you help me to learn more?
(Other definitions for loser that I've seen before include "One mislaying" , "Failed competitor" , "Ineffectual person who regularly fails" , "Roles for the defeated" , "One vanquished" .)