The most common and most likely answer to this crossword clue is the 9 letter word DEADLOCKS
D
E
A
D
L
O
C
K
S
Buddy explains!

The straight part
We know that "knotty problems" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case knotty problems = deadlocks.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 DEADLOCKS mean?
DEADLOCKS noun- A standstill resulting from the opposition of two evenly matched forces; a stalemate or impasse
- An inability to continue due to two programs or devices each requiring a response from the other before completing an operation.
- To cause or to come to a deadlock.