Calibre is a nuisance (4)
I believe the answer is:
bore
I believe this clue is a double definition.
'calibre' is the first definition.
(I know this)
'nuisance' is the second definition.
(I've seen this before)
'is a' is the link.
(Other definitions for bore that I've seen before include "River wave; drill" , "Hollow barrel" , "Calibre of gun - nuisance" , "Drill - pest" , "Person who talks tediously" .)