It's dependent on what's included in the clearance as read by the controller, and where in the world you are:
In UK airports one will generally get "cleared to [airport] via [sid], initial climb [as appropriate, often not included if published], squawk. "
It would then be required that you read all of those components back, and the controller will ask you again for it should you not reply.
Under my VATSIM experience in the DANUBE FAB (BG/RO/MD), it'd be required to read back at minimum sq. code, altitude, sid and QNH (as given at clearance)