Sounds like some quiet cesspool (4)
I believe the answer is:
sump
'cesspool' is the definition.
(synonyms)
'sounds like some quiet' is the wordplay.
'sounds like' indicates a 'sounds like' (homophone) clue.
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'some' is a homophone of 'sum'.
'sum'+'p'='SUMP'
(Other definitions for sump that I've seen before include "Crankcase reservoir" , "Reservoir for oil etc" , "Engine oil reservoir" , "Engine's oil reservoir" , "Oil store" .)