Heavenly room accommodating one in fancy seat (9)
I believe the answer is:
celestial
'heavenly' is the definition.
('celestial' can be a synonym of 'heavenly')
'room accommodating one in fancy seat' is the wordplay.
'room' becomes 'cell' (cell is a kind of room**).
'accommodating' is an insertion indicator.
'one' becomes 'i' (Roman numeral).
'in' means one lot of letters goes inside another.
'fancy' indicates an anagram.
'seat' anagrammed gives 'esta'.
'i' going within 'esta' is 'estia'.
'cell' placed around 'estia' is 'CELESTIAL'.
(Other definitions for celestial that I've seen before include "Relating to the heavens" , "Heavenly" .)