Mural painting (6)
I believe the answer is:
fresco
'mural painting' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for fresco that I've seen before include "Painting done in wet plaster" , "Mode of wall painting" , "Painting using watercolours on plaster" , "Watercolour done on wet plaster" , "Wall-painting" .)