Greatness alternatively in hatred (8)
I believe the answer is:
Here is my best explanation:
'greatness' is the definition.
'alternatively in hatred' is the subsidiary indication.
'alternatively' becomes 'or'.
'in' means one lot of letters goes inside another.
'hatred' becomes 'enmity' (enmity is a kind of hatred).
'or' put within 'enmity' is 'enormity'.
(Other definitions for enormity that I've seen before include "Grave crime or sin", "Great wickedness - more tiny (anag)", "Outrageous deed", "horror", "act of wickedness?", "Great wickedness or huge size", "outrage", "Shocking act", "Extreme wickedness or vastness" and "Outrage".)