Barrel kept in bar, a domed building (7)
I believe the answer is:
rotunda
'domed building' is the definition.
(I've seen this before)
'barrel kept in bar a' is the wordplay.
'barrel' becomes 'tun' (tun is a kind of barrel).
'kept in' means one lot of letters goes inside another.
'bar' becomes 'rod' (I've seen this in other clues**).
'tun' placed into 'rod' is 'rotund'.
'rotund'+'a'='ROTUNDA'
(Other definitions for rotunda that I've seen before include "Circular room or building" , "Round domed room" , "Round domed building" , "Round building or hall" , "domed hall" .)