Not fully developed (8)
I believe the answer is:
immature
'not fully developed' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for immature that I've seen before include "Not yet fully grown or developed" , "Not yet adult or fully developed" , "Undeveloped - unripe" , "behaviour of child" , "Not fully ripe or developed" .)