Beast set up insubstantial northern empire of old (8)
I believe the answer is:
assyrian
'empire of old' is the definition.
(as in the Assyrian Empire)
'beast set up insubstantial northern' is the wordplay.
'beast' becomes 'ass' (I've seen this before).
'set up' says the letters should be written in reverse (in a down clue, letters go up).
'insubstantial' becomes 'airy' (similar in meaning).
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'airy' backwards is 'yria'.
'ass'+'yria'+'n'='ASSYRIAN'
(Other definitions for assyrian that I've seen before include "one in Iraq once" , "ancient language" , "Extinct language" , "The - came down like the wolf on the fold (Byron)" , "race" .)