In league with boy released by Old Bailey (6)
I believe the answer is:
allied
'in league' is the definition.
(thesaurus)
'with boy released by old bailey' is the wordplay.
'with' is a charade indicator (letters next to each other).
'boy' becomes 'al'.
'released by old bailey' becomes 'lied' (I can't explain this - if you can you should give a lot more credence to this answer).
'al'+'lied' is 'ALLIED'.
Can you help me to learn more?
(Other definitions for allied that I've seen before include "(Of nations) working together" , "Owl's cry" , "Joined for mutual benefit" , "in concert" , "Connected, related" .)