Hike; arduous journey (4)
I believe the answer is:
trek
This clue is a double definition.
'hike' is the first definition.
(trekking is a kind of hiking)
'arduous journey' is the second definition.
(I've seen this in another clue)
(Other definitions for trek that I've seen before include "Slog" , "Footslog" , "A long arduous walk" , "A long hard march" , "Long hard journey on foot" .)