Glide or slide over top of mountain (4)
I believe the answer is:
skim
'glide' is the definition.
(I know that skim is a type of glide)
'slide over top of mountain' is the wordplay.
'slide' becomes 'ski' (skis slide over snow).
'over' says to put letters next to each other.
'top of' suggests taking the first letters.
The first letter of 'mountain' is 'm'.
'ski'+'m'='SKIM'
'or' acts as a link.
(Other definitions for skim that I've seen before include "head" , "Move quickly (over a surface)" , "Glance over" , "Remove (cream from milk)" , "Remove top (of milk)" .)