How to fix clipping issue in gltf viewer?

Not really that I’m aware of… and it’s a common issue.

The problem is that the bounding box of the object is computed from the mesh before any animations have been applied. If the animation then moves the mesh outside its original bounding box, you see these kinds of problems.
The renderer can’t automatically compute the bounding box of an animated mesh, so it relies on the original boundingbox being large enough.

You can try to ensure that your original model has all rotation/scaling/position, applied before starting to animate it, and when animating make sure the character isn’t animating outside of its original bounds.
SO for instance… if you started the mesh out at 4x the size you were going to animate it to… that would give you 4x the character size room to move the character around, but still remaining inside its original boundingbox.

If you’re writing a threejs app to use these animations, you often would either disable frustumCulling on them, or assign your own manually defined bounding box.