No, it's back outside Army headquarters (7)
I believe the answer is:
station
'headquarters' is the definition.
(I've seen this before)
'no it's back outside army' is the wordplay.
'back' says the letters should be written in reverse.
'outside' means one lot of letters goes inside another (some letters go outside others).
'army' becomes 'TA' (abbreviation for Territorial Army).
'no'+'its'='noits'
'noits' back-to-front is 'stion'.
'stion' going around 'ta' is 'STATION'.
(Other definitions for station that I've seen before include "Small military base" , "Base, post" , "Broadcasting channel" , "Broadcaster" , "position in society?" .)