Betrayer tsar tries to reform (9)
I believe the answer is:
traitress
'betrayer' is the definition.
(I know that traitress is a type of betrayer)
'tsar tries to reform' is the wordplay.
'to reform' indicates anagramming the letters (letters in a new form).
'tsar'+'tries'='tsartries'
'tsartries' anagrammed gives 'TRAITRESS'.
(Other definitions for traitress that I've seen before include "Betrayer of her country" , "False woman" .)