Tin is needed by the ton in Switzerland (3)
I believe the answer is:
can
'tin' is the definition.
(synonyms)
'needed by the ton in switzerland' is the wordplay.
'needed by' is a charade indicator (letters next to each other).
'the ton' becomes 'c'.
'in switzerland' becomes 'an' (I can't explain this - if you can you should give a lot more credence to this answer).
'c'+'an' is 'CAN'.
'is' acts as a link.
Can you help me to learn more?
(Other definitions for can that I've seen before include "Tin; be able to" , "SECOND PART OF 10" , "Preserve fruit" , "prison" , "Metal container for food" .)