Italian city perversely seen as a source of carbohydrates (6)

'of carbohydrates' is the definition.
The definition suggests a singular noun which matches the answer.

'italian city perversely seen as a source' is the wordplay.
'italian city' becomes 'sugar' (I can't explain this - if you can you should give a lot more credence to this answer).
'perversely' says the letters should be written in reverse (in the converse or wrong direction).
'seen' says to put letters next to each other.
'a source' says to take the initial letters (source can mean a beginning or origin).
The first letter of 'as' is 'a'.
'sugar' written backwards gives 'ragus'.
'ragus'+'a'='RAGUSA'

Can you help me to learn more?