Fight a liberal belief in outdated political system (9)
I believe the answer is:
feudalism
'outdated political system' is the definition.
'feudalism' can be an answer for 'system' (feudalism is a kind of system). I'm unsure of the 'outdated political' bit.
'fight a liberal belief' is the wordplay.
'fight' becomes 'feud' (feuding is a kind of fighting).
'liberal' becomes 'l' (abbreviation).
'belief' becomes 'ism' (the -ism suffix in the names of belief systems).
'feud'+'a'+'l'+'ism'='FEUDALISM'
'in' is the link.
Can you help me to learn more?
(Other definitions for feudalism that I've seen before include "System of lords and vassals" , "System of holding land under owners in middle ages" , "Middle Ages system of holding land by giving service to owner" , "Medieval social system" .)