Windshield treatment (4)
I believe the answer is:
tint
'windshield treatment' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for tint that I've seen before include "Hair dye" , "Shade; colour slightly" , "Quality of colour" , "A slight shade of colour" , "(Softer variety of) colour" .)