Language - one with a Latin derivation (7)
I believe the answer is:
italian
'language' is the definition.
(Italian is a language)
'one with a latin derivation' is the wordplay.
'one' becomes 'i' (Roman numeral).
'with' is an insertion indicator.
'derivation' indicates an anagram.
'latin' anagrammed gives 'tlian'.
'a' placed inside 'tlian' is 'talian'.
'i'+'talian'='ITALIAN'
(Other definitions for italian that I've seen before include "South European" , "Native of Sicily, say" , "Romance language" , "Ain't Ali out of Rome, perhaps?" , "European tongue" .)