Mature trio filling an enclosure (5)
I believe the answer is:
ripen
'mature' is the definition.
(to ripen is to mature)
'trio filling an enclosure' is the wordplay.
'filling' indicates the central letters (the letters which fill the word).
'an enclosure' becomes 'pen' (pen is a kind of enclosure).
The central letters of 'trio' are 'ri'.
'ri'+'pen'='RIPEN'
(Other definitions for ripen that I've seen before include "Become ready to pick" , "Grow to maturity like fruit" , "Become ready for picking" , "Come to fruition" , "Make ready" .)