The most common and most likely answer to this crossword clue is the 8 letter word THRONGED
I've seen this crossword clue in The Daily Telegraph
T
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Buddy explains!

The straight part
We know that "crowded" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case crowded = thronged.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 THRONGED mean?
THRONGED verb- To crowd into a place, especially to fill it.
- To congregate.
- To crowd or press, as persons; to oppress or annoy with a crowd of living beings.