The most common and most likely answer to this crossword clue is the 12 letter word GAINSBOROUGH
I've seen this crossword clue in The Daily Telegraph
G
A
I
N
S
B
O
R
O
U
G
H
More clues leading to the result GAINSBOROUGH
- Artist realises old river branch is crossing
- Royal Academy member wins MP’s constituency
- Painter wins town in local elections
- Artist gets Bob endlessly agitated
- Artist acquires a bit of English capital
- Artist makes sacred tree sketch
- Painter to receive punishment for hooliganism, but not a hooligan
- Artist captures nobody's heart? Harsh
- Artist captures gold in part of tree
- Painter acquires section of New York
Buddy explains!

The straight part
We know that "Painter" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case painter = gainsborough.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.