First three do leap jerkily, as a baby frog (7)
I believe the answer is:
tadpole
'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" .)