Prohibition of financial institution? Not entirely (3)
I believe the answer is:
ban
'prohibition' is the definition.
(synonyms)
'financial institution? not entirely' is the wordplay.
'financial institution?' becomes 'bank' (bank is a kind of financial institution).
'not entirely' means to remove the last letter.
'bank' with its last letter taken off is 'BAN'.
'of' acts as a link.
(Other definitions for ban that I've seen before include "Bar, prohibition" , "'Forbid, prohibit (3)'" , "Exile" , "Embargo" , "Censor or proscribe" .)