This produces measure of sight (5)
I believe the answer is:
optic
'of sight' is the definition.
(I've seen this in another clue)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for optic that I've seen before include "Like a sort of nerve" , "Device for measuring gin etc" , "Relating to the eye" , "Of sight or the eye" , "What could measure the spirits" .)