Appearing unshaven is irrational in high-definition (6)

I believe the answer is:

hispid

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'appearing' is the definition.
Both the answer and definition are adjectives. Maybe they are linked in a way I don't understand?

I cannot understand how the remainder of the clue works.

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(Another definition for hispid that I've seen is " Of animal or plants with stiff coarse hairs".)

I've seen this clue in The Times and The Sunday Times.

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