Hi, Icarus, and welcome,
The answer to the clue for 24ac has to be put into the grid as 18dn, and vice versa.
The clue to 24ac is defined by “insect breeder”. API is a type of computer interface, and ARIST is half of “aristocrat”, one of the best kind.
The clue to 18dn is defined by “cantered around for New York crowd”, implying an American spelling. “United” leads to TIED, which is round PUTT reversed.
Sorry if this is too much of a hint.