Filthy rich boy entertaining old English duke (6)
I believe the answer is:
loaded
'filthy rich' is the definition.
(I've seen this in another clue)
'boy entertaining old english duke' is the wordplay.
'boy' becomes 'lad' (informal term for boy).
'entertaining' means one lot of letters goes inside another (inviting in, as for dinner).
'old' becomes 'o' (abbreviation).
'english' becomes 'E' (abbreviation).
'duke' becomes 'd' (abbreviation).
'lad' enclosing 'o' is 'load'.
'load'+'e'+'d'='LOADED'
(Other definitions for loaded that I've seen before include "Wealthy (slang) - charged" , "drunk- > biased" , "Weighted" , "Wealthy (slang) - drunk (slang)" , "Burdened - with lots of money" .)