This is one of those questions that reappears on the forum from time to time. I though post #2 covered it with reference to the AIM. Cleared for the approach meaning also cleared to descend iaw a published procedure when established on it, but not before. UK CAA CAP 493 has something similar but I do not think it is spelled out in PANS ATM.
The seminal case study being the accident of TWA 514, which descended to platform altitude for an NPA after being cleared for the approach at 44 odd miles, and hit a mountain. Nowadays it is usual in the USA upon receiving approach clearance to also hear the clarification "maintain xx until established".