Promised the return of northern banks (5)
I believe the answer is:
sworn
'promised' is the definition.
(I've seen this before)
'return of northern banks' is the wordplay.
'return of' says the letters should be written in reverse.
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'banks' becomes 'rows' (I've seen this before).
'n'+'rows'='nrows'
'nrows' in reverse letter order is 'SWORN'.
'the' acts as a link.
(Other definitions for sworn that I've seen before include "promised" , "confirmed in office" , "Stated on oath" , "Affirmed" , "Given under oath" .)