Close ranks to preserve banks! (4)
I believe the answer is:
stop
'close' is the definition.
(close can mean to stop or come to an end)
'ranks to preserve banks' is the wordplay.
'banks' says the answer is hidden in the clue (the clue contains or banks the answer).
'STOP' is hidden in the letters of 'ranks to preserve'.
(Other definitions for stop that I've seen before include "Prevent - desist" , "Plug - organ knob - bus halt - period" , "break" , "Post (anag.)" , "colon, say" .)