The most common and most likely answer to this crossword clue is the 7 letter word PLUTEUS
P
L
U
T
E
U
S
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The straight part
We know that "young urchin" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case young urchin = pluteus.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 PLUTEUS mean?
PLUTEUS noun- A low screen between columns, especially one that surrounds the choir of a church
- The free-swimming larvae of echinoderms.