Crossword Genius

Therewith a powdered chalk for example (5,5)

Ross

I believe the answer is:

white earth

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

'powdered chalk for example' is the definition.
I know nothing about this answer so I can't judge whether this works.

'therewith a' is the wordplay.
I cannot quite understand how this works, but
'a' is within the answer.
an anagram of 'therewith' is 'whiteerth' which is within the remaining letters.
No letters remain.
This explanation may well be incorrect...

Can you help me to learn more?

I've seen this clue in The Sunday Times.
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