Reason pet needs treatment for tongue (9)
I believe the answer is:
esperanto
'tongue' is the definition.
(I've seen this before)
'reason pet needs treatment' is the wordplay.
'needs treatment' indicates anagramming the letters (letters must be processed or treated).
'reason'+'pet'='reasonpet'
'reasonpet' anagrammed gives 'ESPERANTO'.
'for' acts as a link.
(Other definitions for esperanto that I've seen before include "Personate (anag.)" , "Universal language" , "A language; neat prose (anag.)" , "Artificial speech-form" , "artificial tongue" .)