Horse starts to amble into northern part of range (8)
I believe the answer is:
mountain
'range' is the definition.
(synonyms)
'horse starts to amble into northern part' is the wordplay.
'horse' becomes 'mount' (mount is a kind of horse).
'starts to' says to take the initial letters.
'part' suggests taking the first letters.
The initial letters of 'amble into' is 'ai'.
The initial letter of 'northern' is 'n'.
'mount'+'ai'+'n'='MOUNTAIN'
'of' acts as a link.
(Other definitions for mountain that I've seen before include "Big hill" , "Projecting land mass" , "Eg, K2" , "Alp" , "Something made out of a molehill?" .)