Immature form of an insect before metamorphosis (5)
I believe the answer is:
larva
'immature form of an insect before metamorphosis' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for larva that I've seen before include "Immature form of caterpillar" , "Caterpillar, for example" , "Tadpole or caterpillar, say" , "Immature stage of an insect" , "nit?" .)