The most common and most likely answer to this crossword clue is the 10 letter word MATCHBOARD
I've seen this crossword clue in The Times
M
A
T
C
H
B
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Buddy explains!

The straight part
We know that "Panels assembled" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case panels assembled = matchboard.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 MATCHBOARD mean?
MATCHBOARD noun- A type of wooden board that connects with others using a tongue and groove system
- (sand casting) A thin piece of material (such as wood, plaster, or metal) that forms and aligns the matched parting surfaces for the two parts (the cope and the drag) of a molding box or flask, to which board patterns are attached in some casting methods.