Thus, a piano is associated with opera (4)
I believe the answer is:
soap
'opera' is the definition.
(thesaurus)
'thus a piano is associated with' is the wordplay.
'thus' becomes 'so' (synonyms).
'piano' becomes 'p' (musical abbreviation).
'is associated with' is a charade indicator (letters next to each other).
'so'+'a'+'p'='SOAP'
(Other definitions for soap that I've seen before include "TV programme; cleanser" , "Bath tablet" , "It has detergent properties" , "Cleaning substance - melodramatic TV or radio series" , "bar in Bath?" .)