1066 was a long time ago. I thought this was a pretty clever clue, but I was in the same boat as you. I am thinking that maybe the original phrase was just 3-3-6, and so it is the first letter of the name that is removed per the rules. Not sure yet as I haven't finished.
I seem to be in a familiar place for me. Relatively straightforward grid fill, but now wasting more time looking for the answer to the question. Does resolving the 7 thematic clues help?
One thing that baffles me a bit is that to spell out a question the extra letter in 4ac surely should be an O, but I have an E. The clue doesn't parse properly unless it's an E, does it? Anyone with me about this?