Mean to maintain maturity (7)
I believe the answer is:
average
'mean' is the definition.
(I know that mean can be written as average)
'maintain maturity' is the wordplay.
'maintain' becomes 'aver' (averring is a kind of maintaining).
'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 "It's common" , "'Medium, middling (7)'" , "Medial" , "representative figure?" , "mean maybe" .)