Immature form of an insect (5)
I believe the answer is:
larva
'immature form of an insect' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for larva that I've seen before include "Insect just out of egg" , "immature six-footer?" , "Immature stage of an insect" , "early form" , "Manage" .)