The most common and most likely answer to this crossword clue is the 9 letter word COSMETICS
I've seen this crossword clue in Independent
C
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The straight part
We know that "Slap" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case slap = cosmetics.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 COSMETICS mean?
COSMETICS noun- Preparations applied externally to change or enhance the beauty of skin, hair, nails, lips, and eyes.
- The study of such products.