Not a bad card for Druid as innervating out a minion the turn before that has 5 attack is something that CAN happen, obviously it doesn't really happen that much but I can see this card maybe seeing some kind of play for experimentation. In Arena it could be really really good however. On curve 3/3 in Arena isn't bad at all and if you get the battlecry off this card will get get great value.
Really really hard to get this effect on curve and because of that, I don't think I can rate this any higher than playable. In fact, because it's so hard to utilize on curve, it can be argued as worse (or as bad) as Second-Rate Bruiser. Of course, later in the game when you can more reliably get the effect to trigger, it becomes much better. So it's more like a 3 mana 5/5 that you likely can't play until at least turn 6+. So yeah, I'll stick with playable.
I'm going to playtest a bit with this card for sure - you'll very rarely activate this effect on turn 3 (and a turn 3 3/3 isn't great but in a pinch not the end of the world) but in the mid-game the tempo gain of dropping this and something else feels powerful - reminds me marginally of thing from below (like a fixed version imo)
I think this is a pretty well-designed carded pushing ramp druid. If you miss your ramp and need a body on the board, a 3/3 for 3 is decent. But, if you ramp up, this card will allow you to cheaply regain your lost tempo. Ramp up, play Ancient of War, then next turn play this card and a spell which will allow you to both play a threat and deal with the board. Cool idea for a card.