You can definitely use Clear(). Just make sure you call SetAlphaEnable(True) on the screen, and that you're passing in a 0 alpha value on the color you're Clear()'ing with.
screen = CreateObject("roScreen", True)
.
.
.
' Enable alpha on the screen
screen.SetAlphaEnable(True)
' Clear the screen, so the video can be seen behind
screen.Clear(&H00000000) ' &H00000000 = 0 = fully transparent black
' Draw your other screen elements
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.
.
screen.SwapBuffers()
Also, make sure you've called SetDestinationRect() on your roVideoPlayer object to specify where the video should be displayed.
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