Flipping work! Drudge penning book, which should pay the bills (9)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

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