Device legally required for serving gin, rum, vodka and whisky in UK pubs since 1963 (5)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
optic
(Other definitions for optic that I've seen before include "Spirits measurer" , "What could measure the spirits" , "Ocular" , "Teacher's maybe dispensed with this" , "Pub measure" .)