The most common and most likely answer to this crossword clue is the 5 letter word APPAL
I've seen this crossword clue in The Daily Telegraph
A
P
P
A
L
More clues leading to the result APPAL
- Dad getting over friend’s outrage
- Horrify a reindeer herder going west
- Shock from head of church shifting a little power to the right
- Pointless request to umpire causes outrage
- Shock answer Northerner returned
- Horrify a friend, parking in the centre
- Disgust American by rolling tongue
- A letter-writer reportedly in shock
- Very quietly brought into AA, left in dismay
- A reindeer herder perhaps repelled revolt
Buddy explains!

The straight part
We know that "shock" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case shock = appal.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does APPAL mean?
APPAL verb- To fill with horror; to dismay.
- To make pale; to blanch.
- To weaken; to reduce in strength
- To grow faint; to become weak; to become dismayed or discouraged.
- To lose flavour or become stale.