Constants used by glClipPlane are in GL2ES1 but glClipPlane is in GL2 whereas it is supported by OpenGL-ES 1 as you can see here: http://www.khronos.org/opengles/sdk/1.1/docs/man/glClipPlane.xml
Actually, only glClipPlanef and glClipPlanex are available in ES 1.0.