Select the color tool and color the background of the cluster transparent (the T in the top right corner of the color picker).
To get rid of the frame, you need to right click it using the color tool and when the color picker is displayed press the space bar. This changes the color selection mode so that you can now color the frame transparent.
If this is too complicated to remember, you can also use the cluster from the classic controls palette, where you can easily color the frame transparent.
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