Long journey on foot (4)
'long 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 "A long arduous journey as in S Africa" , "A long hard march" , "Extended journey" , "Long hard journey on foot" , "Long hike" .)
(Other definitions for hike that I've seen before include "Trek" , "Long walk - sharp increase" , "Long walk; big price rise" , "Walk for pleasure in the country" , "Increase cost" .)