Bank's social entrepreneur is bitter (4)
I believe the answer is:
sour
'is bitter' is the definition.
(I've seen this before)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for sour that I've seen before include "Acid, tart" , "Sharp- or tart-tasting" , "Having an acid taste or smell" , "Acid - rancid" , "Acetic" .)