Doctors so worried with waste matter (5)

I believe the answer is:

dross

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

'waste matter' is the definition.
(dross is a kind of waste matter)

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for dross that I've seen before include "Worthless matter" , "Anything worthless" , "Rubbish, scum" , "Slag!" , "Scum - dregs" .)

I've seen this clue in the Sydney Morning Herald.

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