Substance that makes the skin waterproof (3)
I believe the answer is:
oil
This could be a double definition.
'substance' is the first definition.
(I know that oil is a type of organic non-mineral compound)
'the skin waterproof' is the second definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Perhaps you can see an association between them that I can't see?
'that makes' is the link.
Can you help me to learn more?
(Other definitions for oil that I've seen before include "Crude" , "SECOND PART OF 23" , "What can be extracted" , "eg Olive" , "Fuel, lubricant" .)