Risky banker to spoil one fool by end-of-year deposits (11)

'end-of-year deposits' is the definition.
I don't know anything about this answer so I cannot judge whether it can be defined by this definition.

'risky banker to spoil one fool by' is the wordplay.
I cannot really see how this works, but
'spoil' could be 'mar' (synonyms) and 'mar' is found in the answer.
'one' could be 'o' (I've seen this in other clues) and 'o' is located in the answer.
'by' is present in the answer.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?