Besmirch brand admitting scam, finally (5)
I believe the answer is:
smear
'besmirch' is the definition.
('smear' can be a synonym of 'besmirch')
'brand admitting scam finally' is the wordplay.
'brand' becomes 'sear' (I've seen this before**).
'admitting' indicates putting letters inside.
'finally' indicates one should take the final letters.
The last letter of 'scam' is 'm'.
'sear' enclosing 'm' is 'SMEAR'.
(Other definitions for smear that I've seen before include "Besmirch" , "Spread dirty substance, try to damage reputation" , "Spread a greasy or sticky substance over something" , "False report" , "Slander - tissue sample used for testing" .)