Long arduous walk (4)
I believe the answer is:
trek
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">long arduous walk</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 trek that I've seen before include "Extended journey" , "A long arduous journey as in S Africa" , "Tramp" , "Great journey" , "Long and difficult journey" .)