The most common and most likely answer to this crossword clue is the 7 letter word TEMPERA
I've seen this crossword clue in The Times
T
E
M
P
E
R
A
More clues leading to the result TEMPERA
- Possibly when contracts were short term or medium
- Tone down a painting technique
- One drafted into office takes a long time, painting
- Agency worker takes an age to get paint
- Short term employee has lots of time for painting
- Tone down a painter’s medium
- Composure shown by artist’s first painting technique
- Modify a painting technique
- Do casual work with English artist — he could use it
- Soften a medium for artists
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 = tempera.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.
What does TEMPERA mean?
TEMPERA noun- A medium used to bind pigments in painting, as well as the associated artistic techniques.
- A painting done in this medium.