Being a debt-collector is dull (3)

I believe the answer is:

dun

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'is dull' is the definition.
(thesaurus)

I don't understand how the rest 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" .)

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