Walk up (4)
I believe the answer is:
hike
This clue is a double definition.
'walk' is the first definition.
(hike is a kind of walk)
'up' is the second definition.
(thesaurus)
(Other definitions for hike that I've seen before include "Travel by Shank's Mare" , "Go walking with equipment" , "Long march; increase" , "(Go for) a long walk" , "Long ramble" .)