Crossword Genius

Many a seafarer is a natural swimmer (3)

Ross

I believe the answer is:

dab

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

'swimmer' is the definition.
(dab is a type of fish)

'many a seafarer is a natural' is the wordplay.
I cannot really see how this works, but
'many' could be 'd' (Roman numeral for 500) and 'd' is present in the answer.
'a' is found in the answer.
A single letter 'b' remains which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?

(Other definitions for dab that I've seen before include "Expert; flat fish" , "Fingerprint - fish" , "Gently wipe" , "Press gently; flatfish" , "Apply light touches of colour to the fish" .)

I've seen this clue in the Evening Standard.
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