One with an eye on the target (4)

I believe the answer is:

bull

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'the target' is the definition.
(centre of a target)

I cannot understand how the rest of the clue works.

(Other definitions for bull that I've seen before include "Male elephant, say" , "motion is > nonsense" , "Animal on target?" , "Bovine animal" , "Centre of a target" .)

I've seen this clue in the Evening Standard.

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