Post office with rear gate (7)
I believe the answer is:
postern
'gate' is the definition.
(I know that postern is a type of gate)
'post office with rear' is the wordplay.
'post office' becomes 'po' (abbreviation).
'with' is a charade indicator (letters next to each other).
'rear' becomes 'stern' (the stern is the rear of a boat).
'po'+'stern'='POSTERN'
(Other definitions for postern that I've seen before include "where tradesmen enter?" , "a way in the back" , "Rear entrance" , "Back door eg of castle" , "Back or side entrance" .)