I had the crossers PLU so chose M as my last letter to fit with "choice", but the parsing is given as a DD but I can't find any reference to the rest of the clue under PLUM - have I got it wrong?
Ditto. In the meantime, I'd googled for 200 monkeys plums and of course hit on something that on the face of it looked as if it might just be relevant. But now very glad to see there's a principled established explanation. Even more reassuring is to find that the plum one isn't of cockney origin, so no surprise that it was new to me. https://greensdictofslang.com/entry/qalw4vi