Late getting campanologist a lookalike (4,6)
I believe the answer is:
dead ringer
'a lookalike' is the definition.
(I've seen this before)
'late getting campanologist' is the wordplay.
'late' becomes 'dead' (similar in meaning).
'getting' is a charade indicator (letters next to each other).
'campanologist' becomes 'ringer' (I've seen this before).
'dead'+'ringer'='DEAD-RINGER'
(Other definitions for dead ringer that I've seen before include "Quasimodo at end of novel?" , "Lookalike" , "Someone who looks identical to another (slang)" , "Someone who looks very like another" , "that might explain silent phone call?" .)