Painting on plaster (6)
I believe the answer is:
fresco
'painting on plaster' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for fresco that I've seen before include "Painting style" , "Wall-painting" , "Painting using watercolours on plaster" , "Artwork" , "Method of painting on walls" .)