Crossword Genius

Wax for one red gemstone (4)

Ross

I believe the answer is:

ruby

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

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

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for ruby that I've seen before include "Completely" , "A red jewel" , "Gemstone comprising mainly aluminium oxide" , "killed an assassin" , "Vivid red" .)

I've seen this clue in The Times.
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