Empire once so small and insubstantial to overturn (7)
I believe the answer is:
assyria
'empire once' is the definition.
(ancient empire)
'so small and insubstantial to overturn' is the wordplay.
'so' becomes 'as' (eg 'not as tall as me' means 'not so tall as me').
'small' becomes 's' (abbreviation - e.g. clothes size).
'and' says to put letters next to each other.
'insubstantial' becomes 'airy' (similar in meaning).
'to overturn' shows that the letters should be reversed in order.
'airy' reversed gives 'yria'.
'as'+'s'+'yria'='ASSYRIA'
(Other definitions for assyria that I've seen before include "Ancient Middle Eastern empire" , "old kingdom" , "Nineveh its capital" , "Ancient empire in western Asia" , "Ancient Middle East kingdom - Ira says (anag)" .)