Possible Clues:

• Essential part of officer's report?
• Nut centre; sounds like a rank
• The edible centre of a nut
• Sounds as if he's rank and nutty
• At the core of the corps one is so nutty
• Ear item
• Inner part of a seed or grain
• Sound as if the senior soldier is at the heart of things
• It sounds rank in the middle
• Seed in a hard shell