The most common and most likely answer to this crossword clue is the 9 letter word OBSTRUENT
O
B
S
T
R
U
E
N
T
More clues leading to the result OBSTRUENT
Buddy explains!

The straight part
We know that "causing hold up" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case causing hold up = obstruent.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 OBSTRUENT mean?
OBSTRUENT noun- A consonant sound formed by obstructing the airway, causing turbulence; a plosive, fricative or affricate.
- Anything that obstructs, especially in the passages of the body.
- Causing obstruction; blocking up.