Old boy's treatment producing faint (7)
I believe the answer is:
obscure
'faint' is the definition.
(thesaurus)
'old boy's treatment' is the wordplay.
'old boy's' becomes 'obs' (I've seen this in other clues).
'treatment' becomes 'cure' (cure is a kind of treatment).
'obs'+'cure'='OBSCURE'
'producing' is the link.
(Other definitions for obscure that I've seen before include "Blot out" , "Not clear, indistinct" , "Unclear - recondite" , "Hazy or indistinct" , "Insignificant" .)