Crossword Genius

One calls for a double! (6)


I believe the answer is:


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'for a double' is the definition.
(I've seen this before)

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for ringer that I've seen before include "Doppelganger" , "match" , "Campanologist's" , "American impostor" , "One is almost identical to another in the belfry" .)

I've seen this clue in The Sun.
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