Animal that's a knock-out to the French (5)
I believe the answer is:
koala
'animal' is the definition.
(I have seen 'Australian animal ' mean 'koala' so perhaps 'animal' could also mean 'koala')
'a knock-out to the french' is the wordplay.
'a knock out' becomes 'ko'.
'to the french' becomes 'a la' ('to the' in French).
'ko'+'ala'='KOALA'
'that's' acts as a link.
(Other definitions for koala that I've seen before include "Aussie gum-chewer?" , "One's usually up a tree" , "Tree-dwelling marsupial" , "Australian animal" , "small bear" .)