Being a debt-collector is dull (3)

I believe the answer is:

dun

I'm a little stuck... Click here to teach me more about this clue!

'is dull' is the definition.
(thesaurus)

I cannot understand how the remainder of the clue works.

(Other definitions for dun that I've seen before include "Ask persistently for payment of debt" , "Dull greyish-brown colour" , "Press for payment" , "Dingy brown" , "Greyish brown colour" .)

I've seen this clue in the Evening Standard.

Want a hint initially instead of a full solution? Install my app