Crossword Genius

A lot of rubbish picked up in total (3)

Ross

I believe the answer is:

sum

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'total' is the definition.
(synonyms)

'a lot of rubbish picked up' is the wordplay.
'a lot' becomes 'm'.
'of rubbish' becomes 'us' (I can't explain this - if you can you should give a lot more credence to this answer).
'picked up' says the letters should be written in reverse.
'm'+'us'='mus'
'mus' back-to-front is 'SUM'.

'in' is the link.

Can you help me to learn more?

(Other definitions for sum that I've seen before include "Quantity obtained by addition" , "An additional problem" , "All one has" , "Amount of money; add" , "pupils find it a problem" .)

I've seen this clue in The Independent.
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