Little fish: young child has swapped ring for one (7)

I believe the answer is:

tiddler

'little fish' is the definition.
(tiddler can mean a small fish)

I don't understand the remainder of the clue.

(Other definitions for tiddler that I've seen before include "Minnow" , "Small fish (colloq,)" , "Little chap" , "A small fish children try to catch" , "Small fry" .)

I've seen this clue in The Times.

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