Decay, reportedly beneath and round building (7)
I believe the answer is:
rotunda
'round building' is the definition.
(architectural term)
'decay reportedly beneath' is the wordplay.
'decay' becomes 'rot' (rot is a kind of decay).
'reportedly' indicates a 'sounds like' (homophone) clue.
'beneath' becomes 'under' (under means beneath or underneath).
'under' sounds like 'unda'.
'rot'+'unda'='ROTUNDA'
'and' is the link.
(Other definitions for rotunda that I've seen before include "Italian Pantheon, for instance" , "circular construction" , "Do a turn (anag) -- circular domed building" , "Round domed room" , "Round domed building" .)