The Flame Blade spell description says:
You evoke a fiery blade in your free hand. The blade is similar in size and shape to a scimitar, and it lasts for the duration. lf you let go of the blade, it disappears, but you can evoke the blade again as a bonus action.
You can use your action to make a melee spell attack with the fiery blade. On a hit, the target takes 3d6 fire damage.
Assuming you had some means of holding onto the weapon, could a Moon Druid use Flame Blade after it's cast while Wild Shaped?
Best Answer
Yes, you can use flame blade in Wild Shape...
You can definitely cast the spell and then continue concentrating on it while in wild shape form. Furthermore, you are allowed to take any actions granted to you by the spell (specifically, evoking the blade and attacking with it) while you concentrate on it, thanks to this feature of Wild Shape (emphasis mine):
So just like you can continue using your action to call down lightning bolts while concentrating on call lightning in your wild shape, you can use your action to attack with your flame blade while in wild shape.
... But only if your wild shape can hold a blade
However, the part where you run into problems is
This means that if you wild shape into a wolf, the blade is going to disappear, and even if you evoke it again with a bonus action, it's going to disappear again, because you have no way to hold it in your paws. However, if you were to wild shape into an ape, which has hands with opposable thumbs, I see no reason that you wouldn't be able to wield the blade as normal. Since you retain your wisdom score, you would even use the same attack modifier when attacking with it, since it is a melee spell attack (and not a melee weapon attack).
If you want to turn into a giant owl with flyby and make strafing runs wielding the flame blade in your talons, I'd recommend asking your DM first.