The old boy's remedy is not at all clear (7)
I believe the answer is:
obscure
'not at all clear' is the definition.
(I know that obscure is the opposite attribute of clear)
'the old boy's remedy' is the wordplay.
'the old boy's' becomes 'OBs' (abbreviation for a private school 'old boy').
'remedy' becomes 'cure' (synonyms).
'obs'+'cure'='OBSCURE'
'is' is the link.
(Other definitions for obscure that I've seen before include "Hazy or indistinct" , "incomprehensible" , "Little-known" , "not easily understood" , "Not clearly expressed" .)