Less intelligent one in a party of Americans? (6)
I believe the answer is:
doofus
'less intelligent one' is the definition.
(a doofus is a stupid person)
'party of americans?' is the wordplay.
'party' becomes 'do' (do is a kind of party).
'americans?' becomes 'US' (United States).
'do'+'of'+'us'='DOOFUS'
'in a' is the link.
(Other definitions for doofus that I've seen before include "Nitwit" , "American fool - US food (anag)" .)