Young creature to plead insanity (7)

I believe the answer is:

tadpole

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'young creature' is the definition.

'to plead insanity' is the wordplay.
'to plead' anagrams to 'TADPOLE'.
However, I'm not clear how this is indicated.

Can you help me to learn more?

(Other definitions for tadpole that I've seen before include "being hatched in a puddle?" , "frog may spring from" , "little swimmer" , "young swimmer" , "Frog in the making" .)

I've seen this clue in The Globe and Mail.

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