Quiet lie about mansion (4)
I believe the answer is:
pile
'mansion' is the definition.
(pile can mean a large building)
'quiet lie about' is the wordplay.
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'about' indicates anagramming the letters.
'lie' is an anagram of 'ile'.
'p'+'ile'='PILE'
(Other definitions for pile that I've seen before include "large building" , "Nap of a carpet" , "lot of money" , "Stake" , "Part of reactor" .)