please explain how ClO3- has a tetrahederal shape?

The central atom of ClO₃⁻ ion is Cl atom. The central Cl has 7 valence electrons, and it is linked with 3 oxygen atoms by 1 Cl-O⁻ single bond and 2 Cl=O double bonds. 1 lone pair electrons of the central Cl atom is left unbonded.

The 7 valence electrons form 4 electron pairs surrounding the central Cl atom : 1 Cl-O⁻ single bond, 2 Cl=O double bonds and 1 lone pair of electrons. According to VSEPR Theory, the 4 electron pairs should arranged to be TETRAHEDRAL in shape. The shape of ClO₃⁻ is shown below :
https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcSXjmauFWD_x8NoNvwR5rM3ROsNIdLX9ex9j4gF2tJ32-2wdJXG

However, ClO₃⁻ is usually described as "trigonal pyramidal" in shape, but not "tetrahedral". This is because the lone pair of electrons should not take account in description of the shape of molecule.