Label certain notes “of uncertain authenticity” (7)
I believe the answer is:
dubious
'of uncertain authenticity' is the definition.
(dubious means uncertain or suspicious)
'label certain notes' is the wordplay.
'label' becomes 'dub' (both can mean to give a name).
'certain notes' becomes 'IOUs' ('I owe you' notes).
'dub'+'ious'='DUBIOUS'
(Other definitions for dubious that I've seen before include "Doubtful, uncertain" , "Open to suspicion" , "Uncertain - suspect" , "Of questionable value" , "Unsure, unconvinced" .)