Singing hauliers? (6)

I believe the answer is:

carmen

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'singing' is the definition.
Both the definition and answer are singular nouns.
Maybe you can see a link between them that I can't see?

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for carmen that I've seen before include "Spanish opera by Bizet" , "Musical from Bizet opera" , "Bizet's great Spanish opera" .)

I've seen this clue in the Evening Standard.

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