The answer is obviously 'Regicide' but why? Is 'Icing' an arcane slang word for killing, murder etc? Never come across it. If not that, then why 'regicide?
I really enjoyed this puzzle, which had a few witty definitions - 43 down being a case in point. The use of "ice" or "icing" in this context has been around in the higher end puzzles for as long as I can remember. It is certainly in my CD-ROM version of the BRB dating from 2003.