Malign US agent sent back a third note (6)
I believe the answer is:
defame
'malign' is the definition.
(I know that malign is a more specific form of the action defame)
'us agent sent back a third note' is the wordplay.
'us agent' becomes 'fed' (short for a federal agent).
'sent back' is a reversal indicator.
'third note' becomes 'me' (as in do-re-mi).
'fed' back-to-front is 'def'.
'def'+'a'+'me'='DEFAME'
(Other definitions for defame that I've seen before include "Calumniate, slander" , "'Denigrate, slander (6)'" , "Attack someone's good name" , "Malign" , "Attack good reputation, speak ill of" .)