Decay beneath said building (7)
I believe the answer is:
rotunda
'building' is the definition.
(rotunda is a kind of building)
'decay beneath said' is the wordplay.
'decay' becomes 'rot' (rot can mean to decay).
'beneath' becomes 'under' (under means beneath or underneath).
'said' shows a homophone (sound like).
'under' sounds like 'unda'.
'rot'+'unda'='ROTUNDA'
(Other definitions for rotunda that I've seen before include "circular construction" , "Italian Pantheon, for instance" , "Round building or hall" , "Round domed building" , "Do a turn (anag) -- circular domed building" .)