Fully developed, grown up (6)
I believe the answer is:
mature
I believe this clue is a double definition.
'fully developed' is the first definition.
(I've seen this before)
'grown up' is the second definition.
(I've seen this in another clue)
(Other definitions for mature that I've seen before include "reach the end of term" , "Grow older" , "Age" , "Fully grown and adult" , "Fully-developed" .)