Crossword Genius

Chap leaving aggregate stone (5)

Ross

I believe the answer is:

agate

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'stone' is the definition.
(agate is a type of stone)

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for agate that I've seen before include "Precious stone" , "Decorative stone" , "Semi-precious quartz" , "Old drama critic" , "Quartz with bands of colour, used in jewellery" .)

I've seen this clue in The Times.
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