Crossing border, international ordinance essentially son contravenes (9)

'contravenes' is the definition.
('infringe' can be a synonym of 'contravene')

'crossing border international ordinance essentially son' is the wordplay.
'crossing' is an insertion indicator (to cross can mean to go over or around).
'border' becomes 'fringe' (fringing is a kind of bordering).
'international' becomes 'i' (abbreviation eg in IMF).
'essentially' says to take the centre.
'son' becomes 's' (genealogical abbreviation for son).
The centre of 'ordinance' is 'n'.
'i'+'n'+'s'='ins'
'fringe' going within 'ins' is 'INFRINGES'.