Tramp is some hiker travelling westward (4)
I believe the answer is:
trek
'tramp' is the definition.
(I've seen this before)
'some hiker travelling westward' is the wordplay.
'some' indicates a hidden word (some of the parts of the word are used).
'westward' says the letters should be written backwards.
'kert' is hidden within 'hiker travelling'.
'kert' back-to-front is 'TREK'.
'is' acts as a link.
(Other definitions for trek that I've seen before include "Slog" , "Footslog" , "Long hike" , "Long hard journey on foot" , "Long walk in the mountains?" .)