Earl with his seat in wealthy, men-only parliament (9)
I believe the answer is:
reichstag
'parliament' is the definition.
(I have seen 'European parliament' mean 'reichstag' so perhaps 'parliament' could also mean 'reichstag')
'earl with his seat in wealthy men-only' is the wordplay.
'earl' becomes 'e'.
'with his seat in' indicates putting letters inside.
'wealthy' becomes 'rich' ('rich' can be similar in meaning to 'wealthy').
'men only' becomes 'stag' (I've seen this before).
'e' inserted within 'rich' is 'reich'.
'reich'+'stag'='REICHSTAG'
(Other definitions for reichstag that I've seen before include "Berlin parliamentary building" , "old German house" , "Foreigners assembled" , "European parliament" , "government building" .)