Choose one conservative measure in bar (5)
I believe the answer is:
optic
'measure in bar' is the definition.
(device for measuring alcoholic spirits)
'choose one conservative' is the wordplay.
'choose' becomes 'opt' (both can mean to pick something).
'one' becomes 'i' (Roman numeral).
'conservative' becomes 'C' (UK political abbreviation).
'opt'+'i'+'c'='OPTIC'
(Other definitions for optic that I've seen before include "Relating to the eyes" , "Of sight or the eye" , "Alcoholic drink dispenser" , "Ocular" , "affecting vision?" .)