Monster in two Greek tragedies (4)
I believe the answer is:
ogre
'monster' is the definition.
('ogre' can be a synonym of 'monster')
'in two greek tragedies' is the wordplay.
'in' indicates the answer is hidden within the clue.
'OGRE' can be found hidden inside 'two greek tragedies'.
(Other definitions for ogre that I've seen before include "Evil giant" , "Ergo (anag.)" , "Wicked giant" , "Cruel and wicked person" , "man-eater" .)