Nomadic Sikh passer-by is northbound leaving an important part of old trade route (6,4)
I believe the answer is:
khyber pass
'route' is the definition.
(I have seen 'Mountainous route' mean 'khyber pass' so perhaps 'route' could also mean 'khyber pass')
'nomadic sikh passer-by is northbound leaving an important part of old trade' is the wordplay.
I cannot quite understand how this works, but
an anagram of 'passer' is 'erpass' which is within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for khyber pass that I've seen before include "Mountainous route" , "way across the mountains" , "Asian invasion route" , "Pakistan/Afghanistan border route" .)