Backing for American bank without doubt (6)
I believe the answer is:
'without doubt' is the definition.
(I've seen this before)
'backing for american bank' is the wordplay.
'backing for' says the letters should be written backwards.
'american' becomes 'us' (abbreviation for United States).
'bank' becomes 'rely' (to bank on something is to rely on it).
'us' back-to-front is 'su'.
(Other definitions for surely that I've seen before include "with firmness" , "With certainty" , "Indubitably" , "that's for certain" , "it would seem ironic?" .)