Flat finish (5)
I believe the answer is:
matte
'flat finish' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for matte that I've seen before include "Dull, flat" , "Mask used in film; ET mat (anag.)" , "Mask used to add an image in front of camera" , "Mask used to record two images on same film" , "Unusually dull" .)