Seen in disco: Greek giant (4)
I believe the answer is:
ogre
'giant' is the definition.
(I know that ogre is a type of giant)
'seen in disco greek' is the wordplay.
'seen in' says the answer is hidden in the clue.
'OGRE' is hidden within 'disco greek'.
(Other definitions for ogre that I've seen before include "Evil giant" , "stern one who inspires fear" , "Man-eating monster of fairy tales" , "Terrifying man" , "Monstrous giant" .)