HQ until captured is Parisian fortress (8)
I believe the answer is:
bastille
'parisian fortress' is the definition.
(I've seen this before)
'hq until captured' is the wordplay.
'hq' becomes 'base' (I've seen this before).
'until' becomes 'till' (I've seen this in other clues).
'captured' means one lot of letters goes inside another.
'base' enclosing 'till' is 'BASTILLE'.
'is' is the link.
(Other definitions for bastille that I've seen before include "Building stormed in Paris" , "French fortress" , "Former Parisian prison" , "Ballet is about notorious prison in Paris" , "Paris prison, now opera" .)