a wacko or whacko is a crazy person minus the k or kh. That's my only reasoning pbeach. I'm sure there's more to it. Plus the stuff that went on at Waco was a a bit crazy!
Catron - Wack certainly looks better, but am still struggling with the reasoning :-) Maybe its just a bad clue aka the Telegraph at times... (forgive the pun)
Hi pbeach - like JMR27, I wondered whether part of the clue is missing as I know this was the answer given before and I am not sure it is just coincidence that ACKnowledged has the last three letters of WACK.