The beast Bill went after (7)

I believe the answer is:

buffalo

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'the beast' is the definition.
(type of animal)

I cannot understand how the remainder of the clue works.

(Other definitions for buffalo that I've seen before include "Large, brown, North American animal" , "Large bovine animal" , "Brown bison of North America" , "A hairy" , "creature" .)

I've seen this clue in the Evening Standard.

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