Layer of paint (9)
'layer of paint' is the definition.
(I've seen this in another clue)
This is the entire 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" .)