Does a chlorate ion have resonance structures?
Yes, the chlorate ion has three major contributors to the resonance hybrid.
We can write three important equivalent Lewis structures for the chlorate ion. See this diagram from http://www.austincc.edu/lgregory/Lewis%20Diagram%20polyatomic%20answers.htm
There are other Lewis structures, but they are less important.
Each of the three structures is a resonance contributor.
The structure of the resonance hybrid is
The negative charge is distributed equally among the three O atoms.