To “hedge” a bet means to reduce one’s liabilities (either in full or in part). Bookmakers usually do this by offloading a bet or a group of bets (either in full or in part) with another bookie. It is difficult for individual punters to hedge their bets (other than by making multiple selections on a single event with adjusted stakes). “Acca Edge” seems to be form of “hedging” which Betfair offers on accumulator bets (trebles and above). As you describe, in return for a reduction in odds, you get your stake back should one leg of your accumulator let you down. It is a form of hedging in that you reduce your liabilities (i.e. only two of your three selections in a treble needs to win to see your stake returned) but you pay for it in the form of reduced odds.
So you are certainly correct, dick (and I now understand what the last line in your question means. Edge your Acca = “Edge your accumulator”). But it is, nonetheless, a form of hedging. Whether or not it gives you and “Edge” (as Betfair would like you to believe) is not so clear. Of course if one leg of your treble lets you down you get your stake back and so it seems a good deal. However, if all three win you get a reduced payout so then it might not seem to sao good. As I said in my first answer, it’s all about what you see as the best “value”.