It's easy to see for miles on one (5)
I believe the answer is:
plain
'it's easy' is the definition.
(thesaurus)
'for miles on one' is the wordplay.
'for miles' becomes 'plan' (I can't justify this - if you can you should give a lot more credence to this answer).
'on' is an insertion indicator (as in clothing \'on\' a person).
'one' becomes 'I' (Roman numeral).
'plan' going around 'i' is 'PLAIN'.
'to see' acts as a link.
Can you help me to learn more?
(Other definitions for plain that I've seen before include "Not elaborate; clear" , "Flat land; unmarked" , "stretch of level land" , "Simple, ordinary" , "Not complicated" .)