Fighting vessel (7)
I believe the answer is:
warship
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">fighting vessel</span>' is the definition.<br/ ><span class="explanation-format__explanation">(vessel used in fighting)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for warship that I've seen before include "Fighting vessel" , "Naval asset" , "Eg, destroyer, cruiser" , "It's armed at sea" , "frigate perhaps" .)