Middleman skewers those who are most successful - he's a fiend (3,5)

'middleman skewers those who' is the definition.
The definition and answer can be both people as well as being singular nouns.
Perhaps you can see a link between them that I don't see?

'most successful he's a fiend' is the wordplay.
I cannot quite see how this works, but
'a' is present in the answer.
'most' could be 'best' and 'best' is located in the leftover letters.
'he' is found in the answer.
A single letter 't' remains which might be clued in a way I don't understand.
This explanation may well be incorrect...

'are' is the link.

Can you help me to learn more?