Language may make neat prose (9)
I believe the answer is:
esperanto
'language' is the definition.
(Esperanto is an example)
'make neat prose' is the wordplay.
'make' indicates anagramming the letters.
'neat'+'prose'='neatprose'
'neatprose' anagrammed gives 'ESPERANTO'.
'may' acts as a link.
(Other definitions for esperanto that I've seen before include "No repeats of this artificial language, strangely" , "Language invented by Dr Zamenhof" , "A language; neat prose (anag.)" , "Artificial speech-form" , "Ape tenors (anag.)" .)