The most common and most likely answer to this crossword clue is the 4 plus 3 plus 4 letter word SHOW AND TELL
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Buddy explains!
The straight part
We know that "classroom activity" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case classroom activity = show and tell.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 SHOW AND TELL mean?
SHOW AND TELL noun- A school activity in which each student brings an item to school and gives a talk on it to the class.