Difficult journey on foot (4)
I believe the answer is:
trek
'difficult journey on foot' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for trek that I've seen before include "Long and difficult journey" , "Long walk, expedition" , "A long arduous walk" , "A long arduous journey as in S Africa" , "Long hard journey on foot" .)