Tongue needs iodine applied to back (5)
I believe the answer is:
hindi
'tongue' is the definition.
(Hindi is an example)
'iodine applied to back' is the wordplay.
'iodine' becomes 'I' (chemical symbol).
'applied to' means one lot of letters go next to another.
'back' becomes 'hind' (synonyms).
'i' put after 'hind' is 'HINDI'.
'needs' acts as a link.
(Other definitions for hindi that I've seen before include "Indian language" , "One of India's official languages" , "tongue" , "language many use" , "It's spoken abroad" .)