March split up crossing mountain top (5)
I believe the answer is:
tramp
'march' is the definition.
(I've seen this before)
'split up crossing mountain top' is the wordplay.
'split' becomes 'part' ('part' can be a synonym of 'split'**).
'up' says the letters should be written backwards (in down clue: letters go upwards).
'crossing' is an insertion indicator.
'top' suggests taking the first letters.
The initial letter of 'mountain' is 'm'.
'part' in reverse letter order is 'trap'.
'trap' going around 'm' is 'TRAMP'.
(Other definitions for tramp that I've seen before include "Vagrant, hobo" , "Hike" , "Walk heavily or wearily" , "Derelict - ship" , "Walk heavily like hobo" .)