Claim is so false in one sort of political philosophy (9)
I believe the answer is:
'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.
'claimisso' anagrammed gives 'SOCIALISM'.
'in' is the link.
(Other definitions for socialism that I've seen before include "Left-wing philosophy" , "Labour party belief" .)