Occasionally liable, tax ultimately high liver (4)

I believe the answer is:

ibex

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'high liver' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.

'occasionally liable tax ultimately' is the wordplay.
'occasionally' indicates alternate letters.
'ultimately' indicates one should take the final letters.
The last letter of 'tax' is 'x'.
The alternate letters of 'liable' are 'ibe'.
'ibe'+'x'='IBEX'

Can you help me to learn more?

(Other definitions for ibex that I've seen before include "Pyrenean mountain goat" , "Wild goat with large recurved horns" , "Sure-footed type" , "Mountain animal" , "Large-horned mountain goat" .)

I've seen this clue in The Independent.

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