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It's definitely 'pace'. Well done to Buenchico, showing off your erudition.
Pace fits in a slot ending with a common letter that is also found in several word endings, which I took to be (& is) E. Everything else then works out quite easily.
@happygranny2 - As Buenchico shows with his link, there is a rarely used meaning of 'pace' meaning 'given due deference to' when disagreeing with someone. An example might be, after a Professor says that the Earth is 55% iron, "Pace Professor, but my research shows that it is at least 65% iron." Technically, it is a preposition and is often pronounced 'par-chay', but the pronunciation of latin words is subject to dispute.
I expect mouseymick will be along later with more info on the solution.