Some punctuation is wrong in a shop poster (11)

I believe the answer is:

apostrophes

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

'some punctuation' is the definition.
(I know this)

'wrong in a shop poster' is the wordplay.
'a shop poster' anagrams to 'APOSTROPHES'.
But, I am not clear how this is indicated.

'is' is the link.

Can you help me to learn more?

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