Large gun (6)
I believe the answer is:
cannon
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">large gun</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for cannon that I've seen before include "One of the big guns" , "Large wheeled gun" , "Old naval gun" , "Large gun that fires heavy shells" , "Large artillery piece" .)