Old part in teenage dreams (4)
I believe the answer is:
aged
'old' is the definition.
(similar in meaning)
'part in teenage dreams' is the wordplay.
'part in' indicates a hidden word.
'AGED' can be found hidden inside 'teenage dreams'.
(Other definitions for aged that I've seen before include "Getting on" , "Grew old" , "Antiquated" , "Grew older" , "not like a callow youth" .)