Greasy substance used to waterproof leather (6)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
dubbin
(Other definitions for dubbin that I've seen before include "Tallow mixture used to make leather waterproof" , "It's used to make leather soft and waterproof" , "Thick grease for water-proofing leather" , "Grease for softening leather" .)