Layer of paint (9)
'layer of paint' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for coat that I've seen before include "Overgarment" , "Fleece" , "Ulster, perhaps" , "Jacket" , "Warm outerwear" .)
(Other definitions for undercoat that I've seen before include "tin of paint?" , "The first layer of paint" .)