The most common and most likely answer to this crossword clue is the 3 plus 4 letter word ASH TREE
I've seen this crossword clue in Independent
A
S
H
T
R
E
E
Buddy explains!
The straight part
We know that "source of timber" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case source of timber = ash tree.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.