Possible Clues:

• Islands with shade -- that's the central point
• Chief element provided by openers to nuclear energy
• Central part of a system
• Everything depends on this pitch's quality
• Important notes rewritten as a priority for the policymakers
• Uncertain yes/no taken to show lack of an essential principle to build on
• It's the foundation for crucial one at end of street
• It's essential to follow clues by directions
• Central principle of a policy
• Two things a piano needs — it's the central principle