A post at headquarters (7)
I believe the answer is:
station
This clue is a double definition.
'a post' is the first definition.
(synonyms)
'headquarters' is the second definition.
(I've seen this before)
'at' acts as a link.
(Other definitions for station that I've seen before include "Small military base" , "Stopping-place" , "Broadcasting channel" , "TV or radio channel" , "HQ - stopping place - post" .)