Metamorphosis stage (5)
I believe the answer is:
larva
'metamorphosis stage' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for larva that I've seen before include "Animal in immature state" , "Insect just out of egg" , "Early stage of creature" , "Volcanic outpouring" , "Immature form of an insect before metamorphosis" .)