local POS = Vector(0,0,0) -- some local position (relative to entity)
local ANG = ANgle() -- some local angle (relative to entity)

local bclip = render.EnableClipping(true)


local pos, ang = LocalToWorld(POS, ANG, ent:GetPos(), ent:GetAngles())
local normal = ang:Forward()
render.PushCustomClipPlane(normal, normal:Dot(pos + normal))

-- draw ui

render.EnableClipping(bclip)