Piece of music at last in 4/4 time (5)

I haven't cracked this one yet — but the brilliant team at Times for the Times have:

I wrote in the answer then stared at the clue trying to see a parsing. The best I can offer is it’s a sort of hidden word clue, as 4/4 = ONE and IN ONE TIME includes nonet. If this is correct, it’s pretty devious.