Late campanologist's double (4,6)
I believe the answer is:
dead ringer
'double' is the definition.
(I've seen this before)
'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" .)