Late campanologist's doppelganger (4,6)
I believe the answer is:
dead ringer
'doppelganger' is the definition.
(thesaurus)
'late campanologist's' is the wordplay.
'late' becomes 'dead' (similar in meaning).
'campanologist's' becomes 'ringer' (I've seen this before).
'dead'+'ringer'='DEAD RINGER'
(Other definitions for dead ringer that I've seen before include "Likeness" , "double" , "one looks very much like another" , "Exact image" , "One can do no more changes!" .)