Mythical ship (4)
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">mythical ship</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for argo that I've seen before include "Jason's legendary ship" , "Old ship" , "Transport to find the Golden Fleece" , "Mythological ship" , "Old boat" .)