Harshly made fewer advances in bank (12)
I believe the answer is:
relentlessly
'harshly' is the definition.
(thesaurus)
'fewer advances in bank' is the wordplay.
I cannot really understand how this works, but
'fewer' could be 'less' (associated in meaning) and 'less' is present in the answer.
This may be the basis of clue (or it may be nonsense).
'made' acts as a link.
(Other definitions for relentlessly that I've seen before include "In an unceasingly intense way" , "Inexorably" , "Brooking no resistance" .)