Pressure group in the hall (5)
I believe the answer is:
lobby
I believe this clue is a double definition.
'pressure group' is the first definition.
(I know that pressure group can be written as lobby)
'the hall' is the second definition.
('lobby' can be a synonym of 'hall')
'in' acts as a link.
(Other definitions for lobby that I've seen before include "Vestibule" , "waiting room" , "Antechamber" , "Foyer" , "invoking name of pressure group" .)