Sweet logic on diet -- plum duff! (7,8)

'sweet logic' is the definition.
I don't know anything about this answer so I cannot judge whether it can be defined by this definition.

'diet plum duff' is the wordplay.
I cannot really understand how this works, but
an anagram of 'diet' is 'tied' which is located in the answer.
an anagram of 'plum' is 'umpl' which is located in the answer.
This may be the basis of clue (or it may be nonsense).

'on' is the link.

Can you help me to learn more?