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