Deer has one tongue (5)
I believe the answer is:
hindi
'tongue' is the definition.
(Hindi is an example)
'deer has one' is the wordplay.
'deer' becomes 'hind' (hind is a kind of deer).
'has' is a charade indicator (letters next to each other).
'one' becomes 'i' (Roman numeral).
'hind'+'i'='HINDI'
(Other definitions for hindi that I've seen before include "One of India's official languages" , "Ind. Language" , "Asian language" , "It's spoken abroad" , "language many use" .)