Rely on a financial institution (4)
I believe the answer is:
bank
This clue is a double definition.
'rely' is the first definition.
(thesaurus)
'financial institution' is the second definition.
(bank is a kind of financial institution)
'on a' acts as a link.
(Other definitions for bank that I've seen before include "Count" , "Row" , "Edge (of river) - tier (of oars)" , "Financial establishment" , "Long mound; rely (on)" .)