Harbour vessel (3)
I believe the answer is:
tug
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">harbour vessel</span>' is the definition.<br/ ><span class="explanation-format__explanation">(tugboat)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for tug that I've seen before include "Haul" , "Working boat" , "Pull hard or suddenly" , "Pull like small boat" , "Pull forcibly" .)