Encroach on one northern border (8)
I believe the answer is:
infringe
'encroach on' is the definition.
(I've seen this before)
'one northern border' is the wordplay.
'one' becomes 'i' (Roman numeral).
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'border' becomes 'fringe' (fringing is a kind of bordering**).
'i'+'n'+'fringe'='INFRINGE'
(Other definitions for infringe that I've seen before include "Refining so as to act against rule or agreement" , "Cross boundary" , "Encroach, impinge" , "Break the rules" , "Neglect to obey" .)