Advertisement on northern gate (7)
I believe the answer is:
postern
'gate' is the definition.
(I know that postern is a type of gate)
'advertisement on northern' is the wordplay.
'advertisement' becomes 'poster' (poster is a kind of advertisement).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'poster'+'n'='POSTERN'
(Other definitions for postern that I've seen before include "where tradesmen enter?" , "Back or side entrance" , "Small gate in the rear of a fort or castle" , "Rear entrance" , "private entrance" .)