In sum, a mess-up causing ill-will (6)
I believe the answer is:
animus
'ill-will' is the definition.
(thesaurus)
'in sum a mess-up' is the wordplay.
'mess up' is an anagram indicator (I've seen 'messed up' mean this).
'in'+'sum'+'a'='insuma'
'insuma' with letters rearranged gives 'ANIMUS'.
'causing' acts as a link.
(Other definitions for animus that I've seen before include "Hostility, bad feeling toward" , "Hostile spirit in speech or action" , "Deep-seated hostility or vital force" , "Feeling of strong dislike" , "Hostile feeling" .)