Shady old boy's remedy (7)
I believe the answer is:
obscure
'shady' is the definition.
(thesaurus)
'old boy's remedy' is the wordplay.
'old boy's' becomes 'OBs' (abbreviation for a private school 'old boy').
'remedy' becomes 'cure' (synonyms).
'obs'+'cure'='OBSCURE'
(Other definitions for obscure that I've seen before include "Hazy or cryptic" , "Indistinct; conceal" , "Arcane" , "Vague - mask" , "Enigmatic (like Jude?)" .)