Antagonism shown by heartless adversary carrying it (6)
I believe the answer is:
enmity
'antagonism' is the definition.
('enmity' can be a synonym of 'antagonism')
'heartless adversary carrying it' is the wordplay.
'heartless' says to hollow out the word (remove centre letters).
'adversary' becomes 'enemy' (I've seen this before).
'carrying' is an insertion indicator.
'enemy' with its centre removed is 'enmy'.
'enmy' enclosing 'it' is 'ENMITY'.
'shown by' acts as a link.
(Other definitions for enmity that I've seen before include "Bad blood" , "Hostility, bad feeling towards" , "Personal hostility" , "Hostility, ill-will" , "Hostility between opposing groups or nations" .)