Dull agent demanding payment (3)
I believe the answer is:
dun
This clue is a double definition.
'dull' is the first definition.
(thesaurus)
'agent demanding payment' is the second definition.
(archaic term for a debt collector)
(Other definitions for dun that I've seen before include "Greyish brown colour" , "Dull brownish-grey colour" , "Dull greyish-brown colour" , "Brownish gray" , "Brownish grey horse" .)