Mean to assert maturity (7)
I believe the answer is:
average
'mean' is the definition.
('average' can be a synonym of 'mean')
'assert maturity' is the wordplay.
'assert' becomes 'aver' (averring is a kind of asserting**).
'maturity' becomes 'age' (I've seen this before).
'aver'+'age'='AVERAGE'
'to' is the link.
(Other definitions for average that I've seen before include "Mean, so-so" , "Mean - typical" , "Fair to middling" , "Mediocre - medium" , "Of no exceptional quality or ability" .)