Deck application (5)
I believe the answer is:
stain
'deck application' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for stain that I've seen before include "Penetrative dye; mark" , "Satin (anag.)" , "Stigma" , "Discoloured patch" , "Unsightly mark" .)