Unfortunately 20A is one of the worst clues in this crossword. It has a definition for the whole word (it's a musical term), but the cryptic part is horrible. Start off with a dance of French origin which was popular in the 17th century and cut off the last letter. Put that inside a three letter word for a loud noise. Finally add a two letter word for make, although "party" is the usual cryptic clue for this.
So we have:
"Almost dance", "during racket", "make" (or "party") meaning "decrease in sound".
Hope that helps.