Picture thin coat (4)
I believe the answer is:
film
This clue is a double definition.
'picture' is the first definition.
(I know that picture can be written as film)
'thin coat' is the second definition.
(I've seen this in another clue)
(Other definitions for film that I've seen before include "Skin" , "Cinema entertainment" , "Thin coating for movie." , "Veil; cinema attraction" , "Thin membrane" .)