Calibre (4)
I believe the answer is:
bore
'calibre' is the definition.
(both can mean the hollow part of a gun)
This is the entire clue.
(Other definitions for bore that I've seen before include "Tedious person; steep wave" , "Weary with tediousness" , "Tiresome person - tidal flood" , "Suffered" , "Make a hole with a pointed tool" .)