Expert swimmer, flat out! (3)
I believe the answer is:
dab
'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" .)