Mostly dead -- and popular -- language (5)
I believe the answer is:
latin
'language' is the definition.
(Latin is an example)
'mostly dead and popular' is the wordplay.
'mostly' means to remove the last letter.
'dead' becomes 'late' ('late' can be similar in meaning to 'dead').
'and' means one lot of letters go next to another.
'popular' becomes 'in'.
'late' with its final letter taken off is 'lat'.
'lat'+'in'='LATIN'
(Other definitions for latin that I've seen before include "The language of old Rome" , "Language of old Romans" , "Hispanic, for example" , "foreign language" , "Language of the Caesars" .)