Crossword Genius

Expert swimmer, flat out! (3)

Ross

I believe the answer is:

dab

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

'expert' is the definition.
(I've seen this before)

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for dab that I've seen before include "Press gently; flatfish" , "Expert; light touch" , "Fingerprint - fish" , "Gently wipe" , "Flounder" .)

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