Arduous trip (4)
I believe the answer is:
trek
'arduous trip' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for trek that I've seen before include "A long arduous march" , "Difficult journey on foot" , "Slog" , "A long arduous journey as in S Africa" , "A long arduous walk" .)