Fine plaster for moulding into architectural decorations (6)
I believe the answer is:
stucco
'fine plaster for moulding into architectural decorations' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for stucco that I've seen before include "Mouldable wall plaster" , "Decorative external plaster" , "Fine plaster for ornamenting walls etc" , "Mixture for decorative use on buildings" , "Durable finish for walls" .)