Crossword Genius

Quite dubious, displaying a Hawaiian gesture (5)

Ross

I believe the answer is:

shaka

I'm a little stuck... Click here to teach me more about this clue!

'gesture' is the definition.
Both the definition and answer are singular nouns.
Perhaps you can see an association between them that I don't see?

I don't understand how the rest of the clue works.

Can you help me to learn more?

I've seen this clue in the Sydney Morning Herald.
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