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