Decay reportedly beneath building (7)
I believe the answer is:
rotunda
'building' is the definition.
(rotunda is a kind of building)
'decay reportedly beneath' is the wordplay.
'decay' becomes 'rot' (rot can mean to decay).
'reportedly' indicates a 'sounds like' (homophone) clue.
'beneath' becomes 'under' (under means beneath or underneath).
'under' sounds like 'unda'.
'rot'+'unda'='ROTUNDA'
(Other definitions for rotunda that I've seen before include "Circular building with a dome" , "Round building or hall" , "Round domed room" , "domed hall" , "Circular room or building" .)