Crossword Genius

Startled criminal drops back on all fours (7)

Ross

I believe the answer is:

tetrads

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'fours' is the definition.
('tetrad' can be a synonym of 'four')

I don't understand the rest of the clue.

Can you help me to learn more?

(Another definition for tetrads that I've seen is " Started to get groups of four".)

I've seen this clue in The Independent.
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