Promise breaking taxman (7)
I believe the answer is:
imposer
'taxman' is the definition.
(eg someone imposing taxes)
'promise breaking' is the wordplay.
'breaking' indicates anagramming the letters.
'promise' anagrammed gives 'IMPOSER'.
(Other definitions for imposer that I've seen before include "As enforcer" , "Oppressive type" , "forceful type" , "Person intruding on another's business" .)