Defence ultimately presented by bank employee (4)
I believe the answer is:
dyke
'defence' is the definition.
(a defence against flooding)
'ultimately presented by bank employee' is the wordplay.
'ultimately' says to take the final letters.
The last letters of 'presented by bank employee' are 'DYKE'.
(Other definitions for dyke that I've seen before include "Embankment built to prevent flooding as in Holland" , "defensive wall" , "Dick Van - - (films)" , "Offa's barrier?" , "Ditch; earthwork" .)