Notice attached to rear part of garden gate (7)
I believe the answer is:
postern
'gate' is the definition.
(postern is a kind of gate)
'notice attached to rear part of garden' is the wordplay.
'notice' becomes 'poster' (synonyms).
'attached to' is a charade indicator (letters next to each other).
'rear part of garden' becomes 'n' (I can't justify this - if you can you should believe this answer much more).
'poster'+'n'='POSTERN'
Can you help me to learn more?
(Other definitions for postern that I've seen before include "where tradesmen enter?" , "Back or side entrance" , "Back door eg of castle" , "a way in the back" , "Rear entrance" .)