The most common and most likely answer to this crossword clue is the 8 plus 5 letter word SKIRTING BOARD
I've seen this crossword clue in The Guardian
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Buddy explains!
The straight part
We know that "Piece of wood" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case piece of wood = skirting board.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 SKIRTING BOARD mean?
SKIRTING BOARD noun- (finish carpentry, interior decorating) A panel, normally made of wood, between the floor and the interior wall of a structure, or placed in a position that is subject to repeated knocks