Possible Clues:

• Most important part
• One sounds so rank in the middle of the corps, by the sound of it
• Sounds like the officer at the heart of the corps
• Edible or central part of nut
• Essential part of something an officer mentioned
• Does it sound so rank at the heart of things?
• What a rank-sounding way to be at the centre of things!
• Inner seed of nut
• Sounds as if the officer has got a nut case around him
• Hard centre of nut