The most common and most likely answer to this crossword clue is the 8 letter word WARPAINT
I've seen this crossword clue in The Sunday Telegraph
W
A
R
P
A
I
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T
Buddy explains!
The straight part
We know that "cosmetic" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case cosmetic = warpaint.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 WARPAINT mean?
WARPAINT noun- Pigments ritually applied to the face and/or body (originally and still especially of a Native American warrior) prior to going into a battle.
- Cosmetics, makeup