Right, so butter is the definition here, but it's a bit of a pun — it doesn't mean the stuff you spread, it means something that butts, which gives you a nanny goat. For the wordplay, start with NAN for bread, then tack on NY for the Big Apple, that's New York, and since this is a down clue GOAT goes on top of that – GOAT here being "best ever" as in Greatest Of All Time. String NAN, NY and GOAT together and you're there.