Possible Clues:

• See 66 Across
• Broker
• The Go-Between?
• Part of 10 or 21, or 22 down 14, or 4, 9, 23, or 28
• See 17-Across
• Intermediary no extremist?
• One who may be cut out, and what must be cut out of five answers to make sense of their clues
• It's his business not to be an extremist