Maniac reversing into bar — a domed building (7)
I believe the answer is:
rotunda
'domed building' is the definition.
(I've seen this in another clue)
'maniac reversing into bar a' is the wordplay.
'maniac' becomes 'nut' (I can't explain this - if you can you should give a lot more credence to this answer).
'reversing' says the letters should be written backwards.
'into' means one lot of letters goes inside another.
'bar' becomes 'rod' (both can mean a pole or stick).
'nut' backwards is 'tun'.
'tun' placed into 'rod' is 'rotund'.
'rotund'+'a'='ROTUNDA'
Can you help me to learn more?
(Other definitions for rotunda that I've seen before include "Round domed building" , "Do a turn (anag) -- circular domed building" , "domed hall" , "Italian Pantheon, for instance" , "Round domed room" .)