A lot of rubbish picked up in total (3)
I believe the answer is:
sum
'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" .)