Approach maturity in state prison (5)
I believe the answer is:
ripen
'approach maturity' is the definition.
(ripening can mean becoming mature)
'state prison' is the wordplay.
'state' becomes 'ri' (Rhode Island abbreviation).
'prison' becomes 'pen' (both can mean an enclosure).
'ri'+'pen'='RIPEN'
'in' is the link.
(Other definitions for ripen that I've seen before include "Become ready for picking" , "Come to fruition" , "Grow mature like fruit" , "Mature, age" , "Get ready to eat" .)