Crossword Genius

First three do leap jerkily, as a baby frog (7)

Ross

I believe the answer is:

tadpole

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'baby frog' is the definition.
(I've seen this before)

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for tadpole that I've seen before include "Young Kermit" , "Aquatic larva of frog or toad" , "Immature amphibian" , "Aquatic larva of toads and frogs" , "Baby frog" .)

I've seen this clue in the Sydney Morning Herald.
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