Bad testament to hostility (3,4)
I believe the answer is:
ill will
'hostility' is the definition.
(synonyms)
'bad testament' is the wordplay.
'bad' becomes 'i'll' (associated in meaning).
'testament' becomes 'will' (synonyms).
'ill'+'will'='ILL-WILL'
'to' acts as a link.
(Other definitions for ill will that I've seen before include "Hostility, unkind feeling" , "'Hostility or dislike (3,4)'" , "Positively unfriendly disposition" , "Hostile and unkind feeling" , "Hostile, unfriendly feeling" .)