One may be looking to it reflectively (5)
I believe the answer is:
glass
'one' is the definition.
The answer and definition can be both quantities as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for glass that I've seen before include "Calm lake may be compared to this" , "Eg, tumbler" , "Brittle transparent material" , "Transparent stuff" , "Tumbler or flute?" .)