The most common and most likely answer to this crossword clue is the 9 letter word IMPACTION
I
M
P
A
C
T
I
O
N
More clues leading to the result IMPACTION
Buddy explains!

The straight part
We know that "Complaint" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case complaint = impaction.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 IMPACTION mean?
IMPACTION noun- Compression; the packing together of loose matter
- Something packed together tightly; a mass of densely-packed matter
- A solid, immobile bulk of stool
- Failure of a tooth to erupt into the dental arch, because of adjacent teeth, dense overlying bone, excessive soft tissue, or other factor.