Greek leader relaxed an order (9)
I believe the answer is:
alexander
'greek leader' is the definition.
(Alexander the Great)
'relaxed an order' is the wordplay.
'order' indicates anagramming the letters.
'relaxed'+'an'='relaxedan'
'relaxedan' anagrammed gives 'ALEXANDER'.
(Other definitions for alexander that I've seen before include "Great king of ancient Macedonia" , "A great Greek ruler?" , "The Great conqueror of Greece and Egypt and Persia" , "Field marshal" , "... the Great, ancient Greek king" .)