Admiral's vessel (8)
I believe the answer is:
flagship
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">admiral's vessel</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 flagship that I've seen before include "Pre-eminent product" , "It carries the commander of the fleet" , "Admiral's vessel" , "leader of fleet" , "An organisation's most important thing" .)