The most common and most likely answer to this crossword clue is the 5 letter word GNARL
G
N
A
R
L
More clues leading to the result GNARL
Buddy explains!

The straight part
We know that "wooden lump" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case wooden lump = gnarl.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 GNARL mean?
GNARL noun- A knot in wood; a large or hard knot, or a protuberance with twisted grain, on a tree.
- Something resembling a knot in wood, such as in stone or limbs.
- To knot or twist something.
- Gnarled, knotty, twisted.
- To snarl or growl; to gnar.