The most common and most likely answer to this crossword clue is the 7 letter word SWARMED
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More clues leading to the result SWARMED
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 = swarmed.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 SWARMED mean?
SWARMED verb- To move as a swarm.
- To teem, or be overrun with insects, people, etc.
- To fill a place as a swarm.
- To overwhelm as by an opposing army.
- To climb by gripping with arms and legs alternately.
- To breed multitudes.