How to auto clear other field values when user clears a field

In order to auto populate the service subtype (customized field "subtype") when user selected the service ("affected.item") in Change tickets, in the link "cm3r" I added "Source Field" & "Target Field" in affected.item.

Link: cm3r
Field (From/Source): affected.item
File (To/Target): device
Source Field: [affected.item, subtype]
Target Field: [, subtype]

However, when user clears the value in affected.item, the subtype remains. How to automatically clear the subtype value when user clears the affected.item field?

Thanks a lot.