Caterpillar or tadpole, eg (5)
I believe the answer is:
larva
This clue is a double definition.
'caterpillar' is the first definition.
(I've seen this in another clue)
'tadpole eg' is the second definition.
(I know that tadpole is a type of larva)
(Other definitions for larva that I've seen before include "Worm-like insect before metamorphosis" , "Immature form of caterpillar" , "Grub, caterpillar stage" , "Immature form of (insect) development" , "Caterpillar perhaps" .)