One starting to metamorphose earlier, becomes fully developed insect (5)
I believe the answer is:
imago
'fully developed insect' is the definition.
(I've seen this before)
'one starting to metamorphose earlier' is the wordplay.
'one' becomes 'i' (Roman numeral).
'starting to' indicates taking the first letters.
'earlier' becomes 'ago' (I've seen this before).
The initial letter of 'metamorphose' is 'm'.
'i'+'m'+'ago'='IMAGO'
'becomes' is the link.
(Other definitions for imago that I've seen before include "Last state of insect life" , "For example, monarch ultimately" , "Winged insect -- amigo (anag)" , "Perhaps butterfly" , "subconscious view" .)