Untrustworthy knight promises to repay debts (7)
I believe the answer is:
dubious
'untrustworthy' is the definition.
(I've seen this before)
'knight promises to repay debts' is the wordplay.
'knight' becomes 'dub' (to dub can mean to knight someone).
'promises to repay debts' becomes 'IOUs' (an IOU records a debt).
'dub'+'ious'='DUBIOUS'
(Other definitions for dubious that I've seen before include "Doubtful, not convinced" , "Sceptical, doubtful" , "Open to question" , "Ambiguous or suspect" , "Uncertain - suspect" .)