how to update textfield empty when a different option is selected in single-select list in jira?

[rakesh rocckz]

i have single-select list option. it has two values "A","B" and also i have a multi-line text field.

when i select "A" in the text-fields it shows the values of "A" and when i change the option to "B" it doesnt refresh the text-field the values of "A" stil show in the text-field. how to empty the text field if the single-select list value is changed. please help me out. i am not able to figure out how to fix this.??