Antagonism can be endlessly amusing! (6)
I believe the answer is:
animus
'antagonism' is the definition.
(I've seen this in another clue)
'can be endlessly amusing' is the wordplay.
'can be' indicates anagramming the letters.
'endlessly' means to remove the last letter.
'amusing' with its last letter taken off is 'amusin'.
'amusin' is an anagram of 'ANIMUS'.
(Other definitions for animus that I've seen before include "A feeling of ill will or hostility" , "Heat" , "US man I turn to with hostility" , "Ill-feeling; hatred" , "Motivating spirit" .)