An outrage otherwise concealed by ill will (8)
I believe the answer is:
enormity
'an outrage' is the definition.
(I've seen this before)
'otherwise concealed by ill will' is the wordplay.
'otherwise' becomes 'or'.
'concealed by' indicates putting letters inside.
'ill will' becomes 'enmity' (synonyms).
'or' inserted inside 'enmity' is 'ENORMITY'.
(Other definitions for enormity that I've seen before include "Outrageous act" , "Great wickedness or huge size" , "Atrociousness" , "Evil" , "Grave crime or sin" .)