River Kwai director in bank (4)
I believe the answer is:
lean
'bank' is the definition.
(I've seen this before)
'river kwai director in' is the wordplay.
'river' becomes 'ea' (I've seen this in other clues).
'kwai director' becomes 'ln' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'in' means one lot of letters goes inside another.
'ea' inserted into 'ln' is 'LEAN'.
Can you help me to learn more?
(Other definitions for lean that I've seen before include "Wiry" , "Slim; veer" , "Thin; rest (against)" , "With no fat; slope" , "Inclination" .)