Little known old boy's remedy (7)
I believe the answer is:
obscure
'little known' is the definition.
(I've seen this before)
'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 "Not distinct" , "Conceal; uncertain" , "incomprehensible" , "Little-known" , "Dull, opaque" .)