12 across
Prevent
Shakespeare
forgetting
Bottom
(3)
Prevent Shakespeare forgetting Bottom (3)
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The definition is at the beginning of the clue.
How the clue works
Prevent's your definition there, meaning to bar or block something. Now, Shakespeare's nickname is the Bard, giving you BARD, and "forgetting Bottom" tells you to drop the last letter of that word, leaving BAR.