Immature insect form (5)
I believe the answer is:
larva
'immature insect form' 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 an insect before metamorphosis" , "Woodworm" , "a developmental stage" , "Wriggler" , "Grub of e.g. beetle" .)