Infestation picked up beneath circular building (7)
I believe the answer is:
rotunda
'circular building' is the definition.
(I've seen this in another clue)
'infestation picked up beneath' is the wordplay.
'infestation' becomes 'rot' (I can't justify this - if you can you should believe this answer much more).
'picked up' shows a homophone (sound like).
'beneath' becomes 'under' (under means beneath or underneath).
'under' is a homophone of 'unda'.
'rot'+'unda'='ROTUNDA'
Can you help me to learn more?
(Other definitions for rotunda that I've seen before include "Do a turn (anag) -- circular domed building" , "circular construction" , "Circular room or building" , "Circular building with a dome" , "Round domed room" .)