Crossword Genius

Number three, possibly? (5)

Ross

I believe the answer is:

ether

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

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

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

Can you help me to learn more?

(Other definitions for ether that I've seen before include "Some were put out by this" , "Once widely-used anaesthetic" , "Clear upper air" , "What's clear" , "Chemical solvent" .)

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