The siren goes off, drowning a note by the entertainer (6)
I believe the answer is:
singer
'the entertainer' is the definition.
(singer is a kind of entertainer)
'the siren goes off drowning a note' is the wordplay.
'goes off' indicates an anagram.
'drowning' indicates putting letters inside (inserted letters are drowned or absorbed).
'a note' becomes 'g' (musical note).
'siren' is an anagram of 'siner'.
'siner' enclosing 'g' is 'SINGER'.
'by' acts as a link.
(Other definitions for singer that I've seen before include "Make of sewing machine" , "Vocal performer" , "Member of a choir" , "Musical sounds" , "Crooner" .)