Bill goes to the north gate (7)
I believe the answer is:
postern
'gate' is the definition.
(I know that postern is a type of gate)
'bill goes to the north' is the wordplay.
'bill' becomes 'poster' (synonyms).
'goes to' means one lot of letters go next to another.
'the north' becomes 'N' (abbreviation).
'poster'+'n'='POSTERN'
(Other definitions for postern that I've seen before include "Back or side entrance" , "Back door eg of castle" , "where tradesmen enter?" , "a way in the back" , "Small gate in the rear of a fort or castle" .)