Part of speech in dialect, old language (5)
I believe the answer is:
hindi
'old language' is the definition.
(ancient language)
'part of speech in dialect' is the wordplay.
'part of' indicates a hidden word.
'HINDI' can be found hidden inside 'speech in dialect'.
(Other definitions for hindi that I've seen before include "Language of northern India" , "An official language of India" , "Language derived from Sanskrit" , "It's spoken abroad" , "Indian language" .)