Dull person in the belfry or someone similar (4,6)
I believe the answer is:
dead ringer
'the belfry or someone similar' is the definition.
I know nothing about this answer so I can't judge whether this works.
'dull person' is the wordplay.
'dull' becomes 'dead' (I've seen this before).
'person' becomes 'ringer' (ringer is a kind of person).
'dead'+'ringer'='DEAD-RINGER'
'in' is the link.
Can you help me to learn more?
(Other definitions for dead ringer that I've seen before include "Lookalike" , "Quasimodo at end of novel?" , "that might explain silent phone call?" , "Someone who looks very like another" , "Someone who looks identical to another (slang)" .)