Fortieth head of government to come out of public school (4)

I believe the answer is:

ruby

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'fortieth' is the definition.
(a ruby anniversary marks 40 years)

I don't understand the rest of the clue.

(Other definitions for ruby that I've seen before include "Fortieth anniversary stone" , "A red jewel" , "Gemstone comprising mainly aluminium oxide" , "Girl's name - colour" , "Stony girl" .)

I've seen this clue in The Times.

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