The most common and most likely answer to this crossword clue is the 10 letter word AMBULACRUM
A
M
B
U
L
A
C
R
U
M
Buddy explains!
The straight part
We know that "a bit of an urchin" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a bit of an urchin = ambulacrum.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 AMBULACRUM mean?
AMBULACRUM noun- (of an echinoderm) A row of pores for the protrusion of appendages such as tube feet.