Nickname promises to be questionable (7)
I believe the answer is:
dubious
'be questionable' is the definition.
('dubious' can be similar in meaning to 'questionable')
'nickname promises' is the wordplay.
'nickname' becomes 'dub' ('dub' can be a synonym of 'nickname').
'promises' becomes 'ious' (IOU's, promises to pay a debt).
'dub'+'ious'='DUBIOUS'
'to' acts as a link.
(Other definitions for dubious that I've seen before include "Doubtful, uncertain" , "Unsure, unconvinced" , "Arousing suspicion" , "Doubtful, not convinced" , "Open to question" .)