Possible Clues:

• Instrument major's put on table
• Vitally important directors have a way to enter data
• Manually operated levers on a musical instrument
• Federal Reserve, e.g.?
• (Part of) musical instrument
• One produces notes, which is very important to the directors
• Type like Le Carre's 'absolute friend', by whom tiara got destroyed
• Computer attachment
• Essential directors of electronic instrument
• Vital poet retains old facility for writing