50% off 1 ice cream? (6-4)

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

50% off 1dn  leaves IC, which is not, actually, 99 [it’s XCIX – see here but it’s a lovely clue and I think by now this kind of playing with Roman numerals is accepted in Crosswordland. [I only mentioned it because I knew someone else would.]