In the reaction  $Cl{O}_3^{-}+6H^{+}+6e^{-}\to C{l}^{-}+3H_{2}O$ The Equivalent weight of  $Cl{O}_3^{-}$  is

Equivalent weight of a species in a redox reaction = Formula weight / Change in oxidation state
Oxidation state of ‘Cl’ in the reaction $Cl{O}_3^{-}+6H^{+}+6e^{-}\to C{l}^{-}+3H_{2}O$ changes from  + 5 ( ClO₃^-1) to -1 ( Cl^-1), hence change is 6 units.