Many a seafarer is a natural swimmer (3)
I believe the answer is:
dab
'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" .)