Crossword Genius

Should an animal be sick, will set upon it (6)

Ross

I believe the answer is:

assail

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'should an' is the definition.
I can't judge whether this defines the answer.

'animal be sick will set upon it' is the wordplay.
I cannot quite understand how this works, but
'animal' could be 'ass' (term for a donkey) and 'ass' is found within the answer.
'be sick' could be 'ail' (synonyms) and 'ail' is found within the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?

(Other definitions for assail that I've seen before include "Assault, attack" , "Launch an attack" , "Set upon" , "Attack violently and persistently" , "Aggressively attack" .)

I've seen this clue in the Evening Standard.
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