Assyrian's foe (4)
I believe the answer is:
mede
'assyrian's foe' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for mede that I've seen before include "who proverbially laid down the law" , "Old Indo-European" , "Ancient native of Persia" , "Lawmaker" , "One of the old folk" .)