Devices to measure out alcohol (6)
I believe the answer is:
optics
'devices to measure out alcohol' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for optics that I've seen before include "The scientific study of light and vision" , "Devices to measure out alcohol" , "Spirit measures" , "Field of vision?" , "Study of the physical properties of light" .)