On level south of French border (8)
I believe the answer is:
frontier
'border' is the definition.
(I know that border can be written as frontier)
'on level south of french' is the wordplay.
'level' becomes 'tier' (synonyms).
'south of' says to put letters next to each other (in a down clue, some letters go under others).
'french' becomes 'fr' (abbreviation).
'on'+'tier'='ontier'
'ontier' after 'fr' is 'FRONTIER'.
(Other definitions for frontier that I've seen before include "Limit" , "Marches" , "wild area" , "Land border of a country" , "Outer edge of a settled territory" .)