Bad actor promises to be appalling (9)
I believe the answer is:
atrocious
'be appalling' is the definition.
(I've seen this before)
'bad actor promises' is the wordplay.
'bad' indicates an anagram.
'promises' becomes 'ious' (IOU's, promises to pay a debt).
'actor' anagrammed gives 'atroc'.
'atroc'+'ious'='ATROCIOUS'
'to' is the link.
(Other definitions for atrocious that I've seen before include "Foolish" , "Appallingly bad" , "Extremely bad" , "Horrifyingly wicked" , "Appalling" .)