Full-grown (6)
This clue is a double definition.
'full' is the first definition.
(thesaurus)
'grown' is the second definition.
(similar in meaning)
(Other definitions for mature that I've seen before include "Grown-up" , "reach the end of term" , "Old film star" , "Ripe or fully grown" , "Fully developed physically" .)
'full-grown' is the definition.
(synonyms)
This is all the clue.
(Other definitions for adult that I've seen before include "used to describe dirty movies" , "Blue" , "not fit for children to see" , "No child" , "One no longer young" .)