Answer is: the oxidation number of Cl (chlorine) in chlorate ion is +5.
Chlorate ion (ClO₃⁻) has negative charge.
One oxygen atom in this anion (negatively charged ion) has the oxidation number -2. There are three oxygen atoms in chlorate anion.
x + 3 · (-2) = -1.
x = -1 + 6.
x = +5; the oxidation number of chlorine.