Related Clues:

• Essential to life, most important
• Required or wanted
• A component and indispensable part of, like triangle
• Urgently needed or absolutely necessary
• Wanted, required
• Essential to life, critically important
• Most important, like certain body organs
• Lacked, required
• Altering so as to be necessary to the whole
• Relating to being complete
• Required and wanted
• Lacked and required
• 'Inherent, inbuilt (8)'
• Crucially important
• 'Lacked, required (6)'