Flower from Frank Sinatra wanting Nat King ... (7)

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

FREE (frank, as in “speak freely” = say what you think), then SI [natr] A without NAT and without R (abbreviation for Latin Rex = King). The ellipsis is there because Bluth has split Nat King Cole across two clues, and doesn’t contribute to the wordplay. Flowering plant, related to crocuses and irises.