Immature form of insect (5)
I believe the answer is:
larva
'immature form of insect' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for larva that I've seen before include "Grub of e.g. beetle" , "Immature form of (insect) development" , "E.g. caterpillar form" , "a developmental stage" , "Insect's immature form" .)