how to clear the text field value once we change the option in jira ?

[rakesh rocckz]

single-select list options:   "1stgrade","2ndgrade","3rdgrade"

text field:   

if I select the 1stgrade in the text field it shows the values of the 1stgrade and then again if I select the 2ndgrade it doesn't delete the values of the "1stgrade". how would I clear the text field once I select the different option? please help me out. I tried using setFormvalue(null) but still, it doesn't work.