Outdoor garment (4)
(Other definitions for topcoat that I've seen before include "clothing" , "Ulster?" , "Heavy winter outer garment" , "It's worn on the outside in winter" , "Final layer of paint" .)
'outdoor garment' is the definition.
This is the entire clue.
(Other definitions for coat that I've seen before include "Outer garment" , "Layer of paint" , "blanket" , "Fleece" , "Warm outerwear" .)
(Another definition for donkey jacket that I've seen is " Garment associated with building workers".)