Right, so Number is your definition here, since a cardinal is a type of number. Now, that "19" isn't just a number sitting there — it's pointing you to the answer of clue 19 down, which is EROTIC, and that gives you a synonym CARNAL. Stick DI (that's the Roman numerals for 501) inside CARNAL — "nailing" it, so to speak — and you get CAR-DI-NAL.