On PackageRadar website «Customs clearance in progress» status means these statuses of Korea Post:
Tracking package of Korea Post
With the use of our tracking form you can easily check the status of your package. Also if you want to tracking Korea Post and get your package location updates to your e-mail, you can Register on our website.
Korea Post — is not the only postal service that you can track at PackageRadar. See full list of postal services available.