Claim is so false in one sort of political philosophy (9)
I believe the answer is:
socialism
'one sort of political philosophy' is the definition.
(I know that socialism is a political ideology)
'claim is so false' is the wordplay.
'false' indicates anagramming the letters.
'claim'+'is'+'so'='claimisso'
'claimisso' anagrammed gives 'SOCIALISM'.
'in' is the link.
(Other definitions for socialism that I've seen before include "Left-wing philosophy" , "Labour party belief" .)