Agent upset about most recent smear (7)
I believe the answer is:
plaster
'smear' is the definition.
(I've seen this before)
'agent upset about most recent' is the wordplay.
'agent' becomes 'rep' (short for representative).
'upset' says the letters should be written backwards.
'about' means one lot of letters goes inside another.
'most recent' becomes 'last' (the last in a list is the most recent).
'rep' back-to-front is 'per'.
'per' enclosing 'last' is 'PLASTER'.
(Other definitions for plaster that I've seen before include "Material applied to walls" , "Smooth mixture to cover walls" , "Adhesive material" , "Smooth covering for walls and ceilings" , "Strip of material for dressing wound" .)