Appearing unshaven is irrational in high-definition (6)

I believe the answer is:

hispid

I'm a little stuck... Click here to teach me more about this clue!

'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.

Can you help me to learn more?

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

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

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.