American taking on mature tradition (5)
I believe the answer is:
usage
'tradition' is the definition.
(thesaurus)
'american taking on mature' is the wordplay.
'american' becomes 'us'.
'taking on' is a charade indicator (letters next to each other).
'mature' becomes 'age' ('age' can be a synonym of 'mature').
'us'+'age'='USAGE'
(Other definitions for usage that I've seen before include "Customary practice" , "Established custom" , "Acceptable language" , "Habit" , "linguistic custom?" .)