Long walk or journey (4)
I believe the answer is:
trek
I believe this is a double definition.
'long walk' is the first definition.
(I've seen this in another clue)
'journey' is the second definition.
(I know that journey is a is a kind of of trek)
(Other definitions for trek that I've seen before include "A long arduous journey as in S Africa" , "Expedition or difficult trip" , "Long walk in the mountains?" , "A long hard march" , "Slog" .)