Tongue: deer has one (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' says to put 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" , "Indian language" , "tongue" , "Language of northern India" .)