Possible Clues:

• One is in the lead, but it sound in the middle
• The rank sound of the core for the corps
• Island inhabited by a single soldier
• Klink, e.g.
• Officer's sound heart
• Comissioned military officer
• Sounds as if the officer is at the centre of things
• With 50-Across, he started a fast-food franchise at age 65
• Officer's unit left behind mountain pass
• Blimp or Klink