Stop financial institution closing early (3)
I believe the answer is:
ban
'stop' is the definition.
(I've seen this before)
'financial institution closing early' is the wordplay.
'financial institution' becomes 'bank' (bank is a kind of financial institution).
'closing early' means to remove the last letter.
'bank' with its last letter taken away is 'BAN'.
(Other definitions for ban that I've seen before include "the calling of young vassals" , "Officially prohibit" , "Censor or proscribe" , "Stop" , "Veto" .)