Quarrel over mails being collected by old system (9)
I believe the answer is:
feudalism
'system' is the definition.
(I know that feudalism is a type of social organisation)
'quarrel over mails being collected by old' is the wordplay.
I cannot quite see how this works, but
'quarrel' could be 'feud' (I've seen this in another clue) and 'feud' is present in the answer.
an anagram of 'mails' is 'alism' which is located in the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for feudalism that I've seen before include "Medieval social system" , "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" .)