Hiker tired somewhat over long journey (4)
I believe the answer is:
trek
'long journey' is the definition.
(I've seen this before)
'hiker tired somewhat over' is the wordplay.
'somewhat' says the answer is hidden in the clue.
'over' says the letters should be written backwards.
'kert' is hidden within 'hiker tired'.
'kert' reversed gives 'TREK'.
(Other definitions for trek that I've seen before include "A long arduous walk" , "A long arduous march" , "Long difficult walk" , "Expedition or difficult trip" , "Long walk, expedition" .)