Long hard march (4)
I believe the answer is:
trek
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">long hard march</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 trek that I've seen before include "Long and difficult trip" , "Expedition or difficult trip" , "Slog" , "Long walk, expedition" , "A long arduous march" .)