Experienced mariner (3,3)
I believe the answer is:
sea dog
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">experienced mariner</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 sea dog that I've seen before include "Dosage (anag)" , "'Old salt (3,3)'" , "Ancient Mariner" , "Experienced mariner" , "mythical creature" .)