Back one language (5)
I believe the answer is:
hindi
'language' is the definition.
(Hindi is an example)
'back one' is the wordplay.
'back' becomes 'hind' (synonyms).
'one' becomes 'i' (Roman numeral).
'hind'+'i'='HINDI'
(Other definitions for hindi that I've seen before include "It's spoken abroad" , "Ind. Language" , "Indic language" , "An official language of India" , "Asian language" .)