Crossword Genius

With which to write three Roman numerals? (6)

Ross

I believe the answer is:

pencil

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

'write three roman numerals?' is the definition.
'pencil' can be an answer for 'write' (thesaurus). I am not sure about the 'three roman numerals?' bit.

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

Can you help me to learn more?

(Other definitions for pencil that I've seen before include "Drawing instrument" , "Wooden writer" , "Drawer with graphite in" , "Implement for writing, drawing etc" , "Writing implement with graphite centre" .)

I've seen this clue in The Globe and Mail.
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