The most common and most likely answer to this crossword clue is the 5 letter word NODAL
I've seen this crossword clue in The Guardian
N
O
D
A
L
Buddy explains!

The straight part
We know that "Knotty" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case knotty = nodal.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 NODAL mean?
NODAL adjective- Of the nature of, or relating to, a node.
- Of or relating to the nodus.