Clip when Wc is negative

Is clipping done for negative Wc?
What does it mean when Wc<0?

Technically, negative homogenous components would negate/mirror the vector. (x, y, z, -w) = (-x, -y, -z, w). You should avoid negative homogenous components (both for texture coordinates and vertex positions) as OpenGL might not clip and interpolate those correctly.

Can you pleas direct me where it says that “OpenGL might not clip and interpolate those correctly”

Can Wc be Zero??

Appendix H of the OpenGL Programming Guide under “OpenGL Correctness Tips”:

Avoid using negative w vertex coordinates and negative q texture coordinates. OpenGL might not clip such coordinates correctly and might make interpolation errors when shading primitives defined by such coordinates.
A zero homogenous component is allowed and refers to a vertex at infinity.