One calls for a double! (6)
I believe the answer is:
'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" .)