The post room (6)
I believe the answer is:
This clue is a double definition.
'the post' is the first definition.
('office' can be a synonym of 'post')
'room?' is the second definition.
(I know that office is a type of room)
(Other definitions for office that I've seen before include "Position of authority" , "Place for administrative business" , "Workplace for white-collar workers" , "Room for working" , "Administration room" .)