The most common and most likely answer to this crossword clue is the 7 letter word TIEPOLO
I've seen this crossword clue in The Times
T
I
E
P
O
L
O
More clues leading to the result TIEPOLO
Buddy explains!
The straight part
We know that "He painted frescoes and altarpieces" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case he painted frescoes and altarpieces = tiepolo.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.