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' means one lot of letters go next to another.
'rear part of garden' becomes 'n' (I can't explain 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" .)