Union's leader accommodated in a tidy chamber (6)
I believe the answer is:
atrium
'chamber' is the definition.
(I know that atrium is a type of chamber)
'union's leader accommodated in a tidy' is the wordplay.
'leader' suggests taking the first letters.
'accommodated in' means one lot of letters goes inside another.
'tidy' becomes 'trim' (similar in meaning).
The first letter of 'unions' is 'u'.
'a'+'trim'='atrim'
'u' put within 'atrim' is 'ATRIUM'.
(Other definitions for atrium that I've seen before include "Roofed courtyard" , "Church vestibule" , "Central court of old Roman house, or heart cavity" , "lobby" , "Um, I rat on the chamber of the heart" .)