Attacked animal was unwell (8)
I believe the answer is:
assailed
'attacked' is the definition.
('assail' can be a synonym of 'attack')
'animal was unwell' is the wordplay.
'animal' becomes 'ass' (term for a donkey).
'was unwell' becomes 'ailed' (I've seen this before).
'ass'+'ailed'='ASSAILED'
(Other definitions for assailed that I've seen before include "Went for" , "charged" , "became aggressive" , "Attacked, lashed out" , "Set upon" .)