Begin a voyage (3,4)
I believe the answer is:
set sail
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">begin a voyage</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for set sail that I've seen before include "Leave port (by ship)" , "Embark on a voyage by ship" , "Left port" , "Begin a voyage" , "Leave for (sea) voyage" .)