Crossword Genius

On a roll with navel piercing? (8)

Ross

I believe the answer is:

tumbling

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'on' is the definition.
Although both the answer and definition are adjectives, I cannot understand how they can define each other.

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for tumbling that I've seen before include "Acrobatics" , "Falling down, as if collapsing" , "Falling headlong" , "Quickly declining" , "Rolling over and over, as a gymnast might" .)

I've seen this clue in The Independent.
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