Butter up bisexual partner you've stopped dating (4)

I believe the answer is:

ibex

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'butter' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for ibex that I've seen before include "Mountain goat with curving horns" , "Sure-footed climber" , "Wild goat with double curved horns" , "one seldom puts a foot wrong" , "Wild goat with large recurved horns" .)

I've seen this clue in The Independent.

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