Possible Clues:

• One book might be of service to a number
• First, second, third could be such a numeral
• First is the first one
• Thus I am obstructive and draw the line under pressure
• Numerals such as first and second
• Roman Catholic service book
• A number defining position in series
• Possibly second distracting noise interrupting exam
• First, second or third, e.g.
• Number defining thing's position in a series