Calibre (4)
I believe the answer is:
bore
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">calibre</span>' is the definition.<br/ ><span class="explanation-format__explanation">(both can mean the hollow part of a gun)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(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" .)