Bad guy - no juvenile (5)
I believe the answer is:
young
'juvenile' is the definition.
(I've seen this before)
'bad guy no' is the wordplay.
'bad' indicates an anagram.
'guy'+'no'='guyno'
'guyno' is an anagram of 'YOUNG'.
(Other definitions for young that I've seen before include "no wrinkly!" , "Newly-developed" , "At an early stage of development" , "Mormon pioneer" , "interviewer of PMs on the radio" .)