Amphibian larva (7)
I believe the answer is:
tadpole
'amphibian larva' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for tadpole that I've seen before include "To plead for tiny frog" , "A frog-to-be" , "Larval form" , "Young toad" , "little swimmer" .)