The most common and most likely answer to this crossword clue is the 8 plus 4 letter word ATHLETES FOOT
I've seen this crossword clue in The Daily Telegraph
A
T
H
L
E
T
E
S
F
O
O
T
More clues leading to the result ATHLETES FOOT
Buddy explains!
The straight part
We know that "A complaint that may cause 23 to scratch" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a complaint that may cause 23 to scratch = athletes foot.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.