One from larger family, I’m last to dance such as waltz? (6,4)

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

TRIPLET(one of three children or animals born at the same birth, definitely making the family it belongs to, larger than usual) + I’M + last letter of ( last to ) “ dance ”. Defn: In music, time or rhythm with tree beats to each bar, an example of which is/such as the waltz.