Late campanologist's double (4,6)
I believe the answer is:
dead ringer
'double' is the definition.
(I've seen this in another clue)
'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 "double" , "Exact image" , "that might explain silent phone call?" , "Quasimodo at end of novel?" , "Likeness" .)