Shows maturity in trousers (8)
I believe the answer is:
pageants
'shows' is the definition.
'maturity in trousers' is the wordplay.
'maturity' becomes 'age' (I've seen this before).
'in' indicates putting letters inside.
'trousers' becomes 'pants' ('pants' can be a synonym of 'trousers'**).
'age' put within 'pants' is 'PAGEANTS'.
(Other definitions for pageants that I've seen before include "Elaborate processions or tableaux" , "Dramatic or spectacular tableaux" , "spectacles" , "Rich and spectacular ceremonies" , "ceremonial events" .)