Clue: Flipping work! Drudge penning book, which should pay the bills (9)
Answer: POTBOILER
Explanation: Definition: book, which should pay the bill
definition: a book written (perhaps quickly, or catering to popular trends) to pay the author's bills
 
OP (opus, "work") reversed/"Flipping" + TOILER="Drudge" around B (book)
 
a 'drudge' is someone who does monotonous work