Fellows losing money in dark fabric (5)

I believe the answer is:

denim

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

'fabric' is the definition.
(I've seen this before)

I don't understand the remainder of the clue.

(Other definitions for denim that I've seen before include "Mined (anag)" , "Cotton twill fabric" , "Twill cloth" , "Material for casual trousers or jackets" , "Tough cotton fabric" .)

I've seen this clue in The Independent.

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