Presumptuous fool with posh Chinese porcelain (8)
I believe the answer is:
assuming
'presumptuous' is the definition.
(synonyms)
'fool with posh chinese porcelain' is the wordplay.
'fool' becomes 'ass' (both can mean a stupid person).
'with' means one lot of letters go next to another.
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-u).
'chinese porcelain' becomes 'ming' (Chinese porcelain from the Ming dynasty).
'ass'+'u'+'ming'='ASSUMING'
(Other definitions for assuming that I've seen before include "Presumptuous, arrogant" , "Presupposing" , "Taking to be the case" , "Taking for granted" , "Taking as true" .)