Attack the animal and be ill (6)
I believe the answer is:
assail
'attack' is the definition.
(I know that attack can be written as assail)
'animal and be ill' is the wordplay.
'animal' becomes 'ass' (term for a donkey).
'and' says to put letters next to each other.
'be ill' becomes 'ail' (synonyms).
'ass'+'ail'='ASSAIL'
'the' acts as a link.
(Other definitions for assail that I've seen before include "Sudden attack" , "Attack violently and persistently" , "Criticise severely" , "storm" , "Maybe mug" .)