The most common and most likely answer to this crossword clue is the 3 plus 7 plus 2 plus 3 plus 4 letter word THE MONARCH OF THE GLEN
I've seen this crossword clue in The Times
T
H
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M
O
N
A
R
C
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O
F
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G
L
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N
Buddy explains!
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 = the monarch of the glen.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.