An eye for a drink? (5)
I believe the answer is:
optic
This might be a double definition.
'an eye' is the first definition.
(synonyms)
'drink?' is the second definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'for a' is the link.
Can you help me to learn more?
(Other definitions for optic that I've seen before include "Relating to the eyes" , "bar assistant" , "Of sight or the eye" , "Spirits measurer" , "Device for dispensing measures of spirits" .)