When a Clark-Y airfoil has the base or bottom horizontal, it has a positive angle of attack.
The reason I say this, is because the angle of attack is "usually" measured from the centre of the radius of the leading edge to the center of the trailing edge.
With a Clark-Y having the bottom as a datum, you could have several degrees of angle of attack.
A simple rule is 1/16th of an inch at 4 inches is 1 degree. So if the chord is 12 inches, 3/16ths of an inch is one degree.
What is the height of the centre of the radius of the leading edge, above a straight edge held along the underside of the wing?
See this link for a sample
The horizontal line is displayed at 0 deg AofA. Note the amount the lower surface extends below the centre line at 0 deg.
Note in the right hand upper plot the angle of attack at 0.0 still shows positive lift, even with Reynolds number of only 50,000. When you plot the Re of 100,000 the Coefficient of Lift Cl is much greater at 0deg AofA
In the bottom right graph you see the impact on drag of an increase in the AofA. It starts to increase dramatically at around 10 degrees and goes up steeply with increasing angle.