Ancient language of unwell younger Scotsman (8)
I believe the answer is:
illyrian
'ancient language' is the definition.
(language of the ancient region of Illyria)
'unwell younger scotsman' is the wordplay.
'unwell' becomes 'i'll' (similar in meaning).
'younger' becomes 'yr' (abbreviation used in some aristocratic titles).
'scotsman' becomes 'ian' (common Scottish name).
'ill'+'yr'+'ian'='ILLYRIAN'
'of' acts as a link.
(Other definitions for illyrian that I've seen before include "language from Albania" , "old foreign language" .)