More mature US State Representative retiring (5)
I believe the answer is:
riper
'more mature' is the definition.
('ripe' can be similar in meaning to 'mature')
'us state representative retiring' is the wordplay.
'us state' becomes 'ri' (abbreviation for Rhode Island).
'representative' becomes 'rep' (rep is a kind of representative).
'retiring' shows that the letters should be reversed in order.
'rep' reversed gives 'per'.
'ri'+'per'='RIPER'
(Other definitions for riper that I've seen before include "More mature and ready to be eaten" , "More suitable for picking" , "More ready to pick" , "More ready for picking" , "becomes more mature" .)