Pope endlessly fiddling with his holy principles (10)
I believe the answer is:
philosophy
'principles' is the definition.
(a set of principles or beliefs)
'pope endlessly fiddling with his holy' is the wordplay.
'endlessly' means to remove the last letter.
'fiddling with' indicates an anagram.
'pope' with its last letter taken off is 'pop'.
'his'+'holy'='hisholy'
'pop' mixed up with 'hisholy' gives 'PHILOSOPHY'.
(Other definitions for philosophy that I've seen before include "Platonic affair" , "First P of PPE" , "Outlook" , "Study of the nature of knowledge and reality" , "Study of truth - attitude to life" .)