Advertisement possibly attached to new gate (7)
I believe the answer is:
postern
'gate' is the definition.
(postern is a kind of gate)
'advertisement possibly attached to new' is the wordplay.
I cannot quite see how this works, but
'advertisement' could be 'poster' (poster is a kind of advertisement) and 'poster' is located in the answer.
'new' could be 'n' (common abbreviation eg NT for New Testament) and 'n' is found in the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for postern that I've seen before include "a way in the back" , "Back door eg of castle" , "Small gate in the rear of a fort or castle" , "Rear entrance" , "Back or side entrance" .)