Possible Clues:

• (Of numbers) first or second, perhaps
• Book the First, for One
• Number defining a thing's position in series
• First is one but one isn't one
• Number such as first or second
• Positional number
• Denoting a numerical order in a series
• Maybe first test in which grade lower than C's accepted
• Maybe first exam will be interrupted by unwelcome noise
• Numeral indicating rank order