The most common and most likely answer to this crossword clue is the 10 letter word TENDONITIS
I've seen this crossword clue in The Guardian
T
E
N
D
O
N
I
T
I
S
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 = tendonitis.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 TENDONITIS mean?
TENDONITIS noun- Inflammation of a tendon.