Bully boys grown up perfect (6)
I believe the answer is:
menace
'bully' is the definition.
(thesaurus)
'boys grown up perfect' is the wordplay.
'boys grown up' becomes 'men'.
'perfect' becomes 'ace'.
'men'+'ace'='MENACE'
(Other definitions for menace that I've seen before include "Threat; threaten" , "Threat, show of hostility" , "Intimidate" , "Threat, danger" , "Threaten with violence" .)