It is difficult to describe the characteristics of RP in detail without resorting to precise phonetic (not phonemic) transcription.
I suppose the simplest way of summing it up is to say that it is mainly the articulation of vowel/diphthong sounds that makes any accent, including RP, different from any other.
Probably the only reason some people consider RP to be 'clearer' than many other accents is that it is the BrE accent that is best known throughout the world. Although the accent of a minority of speakers, it was for many years almost the only accent heard on the BBC. and it is still used by many of the teachers and actors who produce recorded EFL materials.