The most common and most likely answer to this crossword clue is the 8 letter word EYELINER
I've seen this crossword clue in The Daily Telegraph
E
Y
E
L
I
N
E
R
More clues leading to the result EYELINER
- Setter heard vessel getting slap on the lid
- Sailor forgetting pop on ship: might one go on the lash?
- Cosmetic overhaul of English river in Sussex town
- It is appealingly drawn on the lid
- Survey vessel’s instrument for enhancing lamps
- Look at ship’s make-up
- Something like a porthole on side of cruise vessel, cosmetic
- Cosmetic damage finally gets the old ship
- Highlight for viewers: grain either side of English river heading West
- Cosmetic watch? Ship back
Buddy explains!

The straight part
We know that "it’s a cosmetic thing" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case it’s a cosmetic thing = eyeliner.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 EYELINER mean?
EYELINER noun- Makeup used to outline the eye, generally applied along or close to the lashline.