A one party language (3)
I believe the answer is:
ido
'language' is the definition.
(Ido is an example)
'a one party' is the wordplay.
'a one' becomes 'i' (Roman numeral).
'party' becomes 'do' (do is a kind of party**).
'i'+'do'='IDO'
(Other definitions for ido that I've seen before include "Words made up" , "Wedding promise" , "See 1" , "International language" , "Required affirmative in wedding ceremony" .)