Ok, two parts to this:
First, I would require you to invoke an aspect related to either an opportunity to hide or the opponent being preoccupied, so he won't see you for a moment. Otherwise, being in fight with someone and then just hiding would be kind of strange.
Second, the way I would do this would depend on the situation.
- Do you want to hide to get out of the fight? That's a concession, you lose the fight, but you live to fight the next one.
- Do you want to attack out of a hidden position? Create an aspect and tag it on your attack. I would probably let you do this without the requirement from the first part, because it would otherwise just be an aspect for an aspect, which wouldn't really make sense.
- Do you want to hide to prepare something without someone hitting you all the time? Put up a stealth block and have the attacker roll alertness "attacks" to find you. Remember that the attacker can do maneuvers as well, to increase his chance of finding you. Of course, this could become option 2a, if your opponent decides to leave, because he can't find you for too long.