Mature part in teenage diary (4)
I believe the answer is:
aged
'mature' is the definition.
(similar in meaning)
'part in teenage diary' is the wordplay.
'part in' indicates the answer is hidden within the clue.
'AGED' can be found hidden inside 'teenage diary'.
(Other definitions for aged that I've seen before include "Old or ancient" , "Grew older" , "Geriatric" , "Long in the tooth" , "Became older" .)