He betrays a characteristic alternative (7)
I believe the answer is:
traitor
'he betrays a' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'characteristic alternative' is the wordplay.
'characteristic' becomes 'trait' (a trait is a quality or characteristic).
'alternative' becomes 'or'.
'trait'+'or'='TRAITOR'
Can you help me to learn more?
(Other definitions for traitor that I've seen before include "Rat displayed" , "Unreliable type" , "Someone guilty of treason" , "Judas" , "Treasonous individual" .)