Somewhat papal, a ceremonial building, impressive (6)
I believe the answer is:
palace
'building impressive' is the definition.
(impressive or majestic building)
'somewhat papal a ceremonial' is the wordplay.
'somewhat' indicates a hidden word.
'PALACE' is hidden within 'papal a ceremonial'.
(Other definitions for palace that I've seen before include "Royal dwelling" , "See 23" , "Regal mansion" , "Official residence of a sovereign" , "Home fit for a king" .)