Teenager changed form (8)
I believe the answer is:
generate
'form' is the definition.
(thesaurus)
'teenager changed' is the wordplay.
'changed' indicates anagramming the letters.
'teenager' with letters rearranged gives 'GENERATE'.
(Other definitions for generate that I've seen before include "Give rise to" , "Produce with green tea" , "Cause to arise; produce" , "Produce (energy, etc)" , "Make" .)