He or I perhaps will lead a railway — that's obvious (10)
I believe the answer is:
elementary
'obvious' is the definition.
Both the answer and definition are adjectives. Maybe they are linked in a way I don't understand?
'he or i perhaps will lead a railway' is the wordplay.
'he or i perhaps' becomes 'element' (I can't justify this - if you can you should give a lot more credence to this answer).
'will lead' means one lot of letters go next to another.
'railway' becomes 'ry' (abbreviation).
'element'+'a'+'ry'='ELEMENTARY'
'that's' acts as a link.
Can you help me to learn more?
(Other definitions for elementary that I've seen before include "like some schools?" , "Simple, introductory" , "description of subatomic particle" , "Simple, my dear Watson!" , "Basic, simple" .)