A natural swimmer, though not great (5)

I believe the answer is:

sprat

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

'a natural swimmer' is the definition.
'sprat' can be an answer for 'swimmer' (sprat is a kind of fish). I'm not certain of the 'a natural' bit.

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for sprat that I've seen before include "'Set a . . . . . to catch a herring'" , "Herring's smaller relative" , "Traps for small fish" , "Parts (anag.)" , "Herring of the east North Atlantic" .)

I've seen this clue in the Evening Standard.

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