Crossword Genius

Butter mountain? The reverse! (4)

Ross

I believe the answer is:

ibex

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This clue could be a double definition.

'butter mountain?' is the first definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.

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

Can you help me to learn more?

(Other definitions for ibex that I've seen before include "one seldom puts a foot wrong" , "Wind goat of North Africa" , "Wild mountain goat" , "one used to high living" , "Large-horned mountain goat" .)

I've seen this clue in The Times.
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