Cryptic crossword clue
Painting is first bit of art to be protected by metal bars 9

The most common and most likely answer to this crossword clue is the 9 letter word GRISAILLE

I've seen this crossword clue in Financial Times
G
R
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S
A
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The straight part
We know that "Painting" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case painting = grisaille.
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

GRISAILLE noun
  1. In painting, a method of working which employs only varying values of gray to create form. Often a preliminary step in a fully colored painting.
  2. A stained-glass window in this style.

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