No great swimmers, but popular with some youngsters (8)

I believe the answer is:

tiddlers

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'some youngsters' is the definition.
('tiddler' can be a synonym of 'youngster')

I don't understand the rest of the clue.

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(Other definitions for tiddlers that I've seen before include "Small fish, minnows say" , "Very small fish or unimportant things" , "Small fishes" , "little fishes" .)

I've seen this clue in the Evening Standard.

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