Hail at sea (4)
I believe the answer is:
ahoy
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">hail at sea</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 ahoy that I've seen before include "Mariner's challenge" , "Hail at sea" , "Nautical shout" , "Sailor's call for attention" , "Coastal land" .)