Fortieth curry for Cockneys? (4)

I believe the answer is:

ruby

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But I can't explain it at all! Could you help me understand?

(Other definitions for ruby that I've seen before include "Deep 6 across" , "killed an assassin" , "Gemstone - shade of red" , "Deep red variety of corundum" , "Completely" .)

I've seen this clue in The Sun.

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