Hack: A common drudge; especially a literary drudge; hence a poor writer, a mere scribbler.
Hack: A half-breed horse with more bone and substance than a thoroughbred.
Hack (as a verb): To make rough cuts, to deal cutting blows.

Phillip Knightley prefaces his book with these definitions, so which does he want to identify himself with? Surely not the first. A mere scribbler he may have been early in his career, especially when he was recycling other journalists’ stories (hacking them about, perhaps?) at the London officer of the Australian Daily Mirror. But no-one, now, could call him a poor writer.