import bpy

frame = bpy.context.scene.frame_current

#bpy.types.UserPreferencesSystem.gl_clip_alpha(0.1)

def animathing(scene):
    def main(context):
        frame = bpy.context.scene.frame_current
        prefs = context.user_preferences.system
        # Want clipping to be 0 at 450, 0.1 at 550.
        clipping = ((float(frame) - 450) / 100.0)*0.025
        print(clipping) 
        prefs.gl_clip_alpha = float(clipping)
        
    main(bpy.context)
     
 
 # Run the script every time the frame has changed        
bpy.app.handlers.frame_change_pre.append(animathing)    
     
#main(bpy.context)