Italian lady missing son married American dullard (9)
I believe the answer is:
ignoramus
'dullard' is the definition.
(thesaurus)
'italian lady missing son married american' is the wordplay.
'italian lady' becomes 'signora' (I've seen this before).
'missing' suggests deleting specific letters.
'son' becomes 's' (genealogical abbreviation for son).
'married american' becomes 'mus' (I can't explain this - if you can you should give a lot more credence to this answer).
'signora' with 's' removed is 'ignora'.
'ignora'+'mus'='IGNORAMUS'
Can you help me to learn more?
(Other definitions for ignoramus that I've seen before include "Imbecile" , "I wouldn't know" , "At the present time" , "Ignorant person" , "Dimwit" .)