A superior assistant in a bank job? (7)

I believe the answer is:

abetter

I'm a little stuck... Click here to teach me more about this clue!

'assistant in a bank job?' is the definition.
'abetter' can be an answer for 'assistant' (I've seen this before). I am not sure about the 'in a bank job?' bit.

'a superior' is the wordplay.
'superior' becomes 'better' (better is a kind of superior).
'a'+'better'='ABETTER'

Can you help me to learn more?

(Other definitions for abetter that I've seen before include "who gives encouragement?" , "He criminally assists" , "He helps criminal" , "associate" , "One who incites" .)

I've seen this clue in The Independent.

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