Old boy's remedy may be unfamiliar (7)
I believe the answer is:
obscure
'unfamiliar' is the definition.
(thesaurus)
'old boy's remedy' is the wordplay.
'old boy's' becomes 'obs' (I've seen this in other clues).
'remedy' becomes 'cure' ('cure' can be a synonym of 'remedy').
'obs'+'cure'='OBSCURE'
'may be' acts as a link.
(Other definitions for obscure that I've seen before include "Indistinct; conceal" , "Unheard-of" , "Vague - mask" , "Dim, vague or not well known" , "Make unclear, indistinct, or blurred" .)