Old Paul from the Beatles mostly upheld historic philosopher (5)
I believe the answer is:
occam
'philosopher' is the definition.
(I've seen this before)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for occam that I've seen before include "Man with razor" , "Oxford scholar" , "One scientific principle is -'s razor" , "English philosopher, William (with a Razor!), d. 1349" , "medieval philosopher" .)