The most common and most likely answer to this crossword clue is the 9 letter word UNDERCOAT
I've seen this crossword clue in The Times
U
N
D
E
R
C
O
A
T
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The straight part
We know that "Paint" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case paint = undercoat.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.
Crossword Answer definitions
UNDERCOAT noun- A layer of short hairs underneath the longer ones of an animal's fur
- A coat of paint or other material applied onto a surface before that of a topcoat; a coloured primer
- A coat for wearing indoors, under an overcoat.
- To apply an undercoat to.