Can you be more precise:
Where is the bitmap ?
On the Activity or also on the ImageView ?
What do you mean with but now how to clear the fretBoardCanvas?
If I am not mistaken, the canvas.bitmap is updated only when you initialize it. This means that the canvas.bitmap does not include the circle, although it drew it on the imageview.
Try to do drawbitmap by the canvas, using its own bitmap and see what happens (I have not tried what I suggest so I'm not sure it works).
A Canvas is not a layer above the ImageView View, but 'only' a class of drawing routines that are referenced to a View's bitmap ot to a bitmap.
I could suggest you 2 possibilities:
In your example, add a bitmap with the original image.
Draw the circles as you did on the ImageView.
To erase the circles, set the ImageView's image to the original bitmap.
Add a transparent Panel over the ImageView.
Draw the circles onto the Panel.
To erase the circles, set the Panel's colot to transparent.
Attached a small test program showing both possibilities.