Possible Clues:

• Squiggly diacritic
• Mark over an 'n'
• Mark used four times in this puzzle's solution
• Mark from Madrid
• Mark's hat being worn by daughter
• What senor should have — daughter wearing hat
• Accent over an "n"
• After reflection, change what's written about learner's Spanish mark
• It's displayed on some letters until December
• Symbol of equivalence, in math