Horrify a friend, parking in the centre (5)

I believe the answer is:

appal

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

I don't understand the rest of the clue.

(Other definitions for appal that I've seen before include "To shock, dismay" , "expressed dismay" , "Dismay, horrify" , "Fill with horror" , "Outrage" .)

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

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