Bar measure? Ring officer about it going up (5)
I believe the answer is:
optic
'bar measure?' is the definition.
(I've seen this before)
'officer about it going up' is the wordplay.
'officer' becomes 'co' (Commanding Officer abbreviation).
'about' indicates putting letters inside.
'going' becomes 'p' (I can't justify this - if you can you should believe this answer much more).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'it'+'p'='itp'
'co' placed around 'itp' is 'citpo'.
'citpo' back-to-front is 'OPTIC'.
'ring' is the link.
This may not be correct. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for optic that I've seen before include "bar assistant" , "Of the eye - drinks dispenser" , "Body part" , "Pertaining to the eye" , "Alcoholic drink dispenser" .)