Kind of glass used for measuring drinks? (7)
I believe the answer is:
optical
'kind of glass' is the definition.
(I've seen this in another clue)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for optical that I've seen before include "Concerning sight" , "Coal tip (anag)" , "To do with sight, perhaps an illusion" , "Relating to the eye" , "Visual" .)