Define internal signal as a base clock?

You can route the input to an output and back in through another pin but not internally.

You've to route your signal to an output then connect the output to an input (maybe choose a dedicated fast input line or a global one) with an external wire.

It's a very annoing problem that I've in almost all design with Quartus.

Sometimes I've had success in put a global buffer out of the mux and then giving it assignement as a clock (and before cut the timing to the source of the mux).

The problem is that sometimes it works and sometimes not and I can't understand why.