They're undrinkable (6)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
solids
I'm afraid I can't explain this answer.
Can you help me to learn more?
(Other definitions for solids that I've seen before include "Three-dimensional objects" , "They're not hollow" , "Bodies or shapes with three dimensions" , "Food, as opposed to drinks" , "Three-dimensional shapes" .)