The most common and most likely answer to this crossword clue is the 10 letter word GONORRHOEA
I've seen this crossword clue in The Guardian
G
O
N
O
R
R
H
O
E
A
Buddy explains!
The straight part
We know that "Clap" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case clap = gonorrhoea.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 GONORRHOEA mean?
GONORRHOEA noun- An STD caused by a species of bacteria (the gonococcus) that affects the mucous membrane of the genital and urinary tracts.