Horrify a learner on TikTok or Instagram, say (5)

I believe the answer is:

appal

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

'horrify' is the definition.
(I know that horrify can be written as appal)

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for appal that I've seen before include "Cause to be unpleasantly surprised" , "Scandalise" , "To shock, dismay" , "Put at rest (suspicion)" , "Outrage" .)

I've seen this clue in The Times.

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