Painting on plaster (6)
I believe the answer is:
fresco
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">painting on plaster</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(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" .)