One indicating ratio of emigrant group without base (5)

I believe the answer is:

colon

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'one indicating ratio' is the definition.
(a colon separates the two numbers in a ratio)

I don't understand the rest of the clue.

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(Other definitions for colon that I've seen before include "Costa Rican currency" , ": - bit of the gut" , "City at the Caribbean entrance to the Panama Canal" , "Part of intestines" , "Large intestine; punctuation mark" .)

I've seen this clue in The Telegraph.

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