Working of active clamp....?
In order for Vout to be -10V, the op amp would have to output below -20.7V, since the input is at least +10V.

But that's not how op amps work.

In the presence of feedback, the op amp will output the exact voltage required to (approximately) equalize both its inputs. In order to bring the inverting input to at most +10V in order to match the non-inverting input, it needs to output at most +9.3V. The diode will therefore remain forward-biased when the input is greater than +10V, and be reverse-biased or at equilibrium when it is below that.