Thus, a quiet detergent (4)
I believe the answer is:
soap
'detergent' is the definition.
(I've seen this before)
'thus a quiet' is the wordplay.
'thus' becomes 'so' (synonyms).
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'so'+'a'+'p'='SOAP'
(Other definitions for soap that I've seen before include "TV programme - sudsy bar" , "If soft, it's used to flatter" , "TV programme; cleanser" , "Sudsy TV show?" , "TV entertainment" .)