Stone arguably regularly deficient (4)

I believe the answer is:

ruby

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'stone' is the definition.
(I've seen this in other clues)

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

(Other definitions for ruby that I've seen before include "Gemstone comprising mainly aluminium oxide" , "Precious red gem" , "Girl's name - colour" , "Deep red variety of corundum" , "A red jewel" .)

I've seen this clue in The Times.

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