Antagonism causes me tiny trouble (6)
I believe the answer is:
enmity
'antagonism' is the definition.
(I know that antagonism can be written as enmity)
'me tiny trouble' is the wordplay.
'trouble' is an anagram indicator.
'me'+'tiny'='metiny'
'metiny' with letters rearranged gives 'ENMITY'.
'causes' acts as a link.
(Other definitions for enmity that I've seen before include "Hostility between opposing people or countries" , "Hostility between opposing groups or nations" , "Antagonism, ill-will" , "ill will" , "Personal hostility" .)