Mural painted on wet plaster (6)
I believe the answer is:
fresco
'mural painted on wet plaster' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for fresco that I've seen before include "Method of painting on walls" , "Work of art" , "Artistic work" , "Mural painting" , "Mural painted on a wall" .)