GI in Scots mountain makes start (5)
I believe the answer is:
begin
'start' is the definition.
(synonyms)
'gi in scots mountain' is the wordplay.
'in' means one lot of letters goes inside another.
'scots mountain' becomes 'ben' (eg Ben Nevis).
'gi' going into 'ben' is 'BEGIN'.
'makes' is the link.
(Other definitions for begin that I've seen before include "Partly open" , "Make a start" , "Launch" , "former Israeli Nobel peace prize winner" , "Found" .)