Advertisement in front of northern gate (7)
I believe the answer is:
postern
'gate' is the definition.
(I know that postern is a type of gate)
'advertisement in front of northern' is the wordplay.
'advertisement' becomes 'poster' (poster is a kind of advertisement).
'in front of' indicates taking the first letters.
The first letter of 'northern' is 'n'.
'poster'+'n'='POSTERN'
(Other definitions for postern that I've seen before include "Rear entrance" , "Small gate in the rear of a fort or castle" , "Back door eg of castle" , "Back gate" , "private entrance" .)