Campanologist's double? (6)
I believe the answer is:
ringer
This clue is a double definition.
'campanologist's' is the first definition.
(I've seen this before)
'double?' is the second definition.
(I know that ringer is a type of double)
(Other definitions for ringer that I've seen before include "clanger?" , "Doppelganger" , "Campanologist - spitting image" , "American impostor" , "match" .)