The most common and most likely answer to this crossword clue is the 11 letter word FOUR-BY-TWO
I've seen this crossword clue in The Times
F
O
U
R
-
B
Y
-
T
W
O
Buddy explains!
The straight part
We know that "Piece of timber" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case piece of timber = four-by-two.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.