With such a sum, you're not badly off (6)

I believe the answer is:

goodly

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'with such a sum you're' is the definition.
I can't tell whether this definition defines the answer.

'not badly off' is the wordplay.
'not badly' becomes 'good' ('good' is the opposite to 'badly').
'off' becomes 'ly' (I can't justify this - if you can you should give a lot more credence to this answer).
'good'+'ly'='GOODLY'

Can you help me to learn more?

(Another definition for goodly that I've seen is " Excellent, virtuous (arch.), considerable (quantity)".)

I've seen this clue in the Evening Standard.

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