Man-eater's so stuck up (4)
I believe the answer is:
ogre
'man-eater's' is the definition.
(I've seen this before)
'so stuck up' is the wordplay.
'so' becomes 'ergo' (synonyms).
'stuck up' says the letters should be written backwards.
'ergo' back-to-front is 'OGRE'.
(Other definitions for ogre that I've seen before include "Terrifying monster" , "Ugly giant" , "stern one who inspires fear" , "Wicked giant" , "Bugbear" .)