Riddle somewhat improper for a teenager (9)
I believe the answer is:
perforate
'riddle' is the definition.
(thesaurus)
'somewhat improper for a teenager' is the wordplay.
'somewhat' indicates the answer is hidden within the clue.
'PERFORATE' is hidden in the letters of 'improper for a teenager'.
(Other definitions for perforate that I've seen before include "Make hole or holes through" , "punch" , "pass through hole" , "Puncture" , "bore!" .)