Rising of Eastern enemy (3)
I believe the answer is:
foe
'enemy' is the definition.
(synonyms)
'rising of eastern' is the wordplay.
'rising' shows that the letters should be reversed in order (in a down clue, letters go up).
'eastern' becomes 'e'.
'of' backwards is 'fo'.
'fo'+'e'='FOE'
(Other definitions for foe that I've seen before include "Someone opposing" , "friend never!" , "Opposition" , "Evil rival" , "Antagonist" .)