Use imperial measurement for liquid and middle-sized crystal, perhaps (6)
I believe the answer is:
quartz
'use' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for quartz that I've seen before include "Hard mineral, crystal in watch" , "Silicon dioxide used as a gemstone" , "Hard glossy mineral found in rocks" , "Hard silica mineral" , "Colourless/white mineral" .)