Crossword Genius

Sum advanced by Chelsea's Mason? (6)

Ross

I believe the answer is:

amount

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

'sum' is the definition.
(synonyms)

'advanced by chelsea's mason?' is the wordplay.
I cannot quite see how this works, but
'advanced' could be 'a' and 'a' is present in the answer.
The remaining letters 'mount' is a valid word which might be clued in a way I don't understand.
This may be the basis of clue (or it may be nonsense).

Can you help me to learn more?

(Other definitions for amount that I've seen before include "value" , "Add up or be equivalent to" , "Measure, quantity" , "Total or extent" , "Size" .)

I've seen this clue in The Sun.
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