Basis of philosophy exists with every single PM (9)
I believe the answer is:
taoiseach
'pm' is the definition.
(I've seen this before)
'basis of philosophy exists with every single' is the wordplay.
'basis of philosophy' becomes 'tao' (I can't justify this - if you can you should believe this answer much more).
'exists' becomes 'is' (if something 'is', it exists).
'with' means one lot of letters go next to another.
'every single' becomes 'each' (I've seen this in another clue).
'tao'+'is'+'each'='TAOISEACH'
Can you help me to learn more?
(Other definitions for taoiseach that I've seen before include "I 'ate chaos (anag) - Irish Republic's prime minister" , "Irish premier" , "Western PM" , "PM in Dublin" , "Irish head of government" .)