For the sack in shake up following initial upheaval (10)

I believe the answer is:

aftershock

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'upheaval' is the definition.
(aftershock is a kind of upheaval)

'for the sack in shake up following initial' is the wordplay.
'for the sack' can be anagrammed to 'AFTERSHOCK'.
However, I am unsure how the anagram is indicated.

Can you help me to learn more?

(Other definitions for aftershock that I've seen before include "Tremor following earthquake" , "Ground-breaking" , "that causes another little wobble" , "What might have cracked open house" , "gives one the shakes later" .)

I've seen this clue in The Irish Times.

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