Thus, a quiet serial (4)
I believe the answer is:
soap
'serial' is the definition.
(thesaurus)
'thus a quiet' is the wordplay.
'thus' becomes 'so' ('so' can be a synonym of 'thus').
'a quiet' becomes 'ap' (I've seen this in other clues).
'so'+'ap'='SOAP'
(Other definitions for soap that I've seen before include "Cleansing TV serial drama" , "TV programme; wash" , "Cleaner - TV programme" , "Washing substance" , "Cleaning agent" .)