One may look into it reflectively (5)
I believe the answer is:
glass
'one' is the definition.
The definition and answer can be both quantities as well as being singular nouns.
Maybe you can see an association between them that I can't see?
'look into it reflectively' is the wordplay.
I cannot really understand how this works, but
'look' could be 'la' (I've seen this in other clues) and 'la' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
'may' is the link.
Can you help me to learn more?
(Other definitions for glass that I've seen before include "Tumbler, mirror" , "Balloon, say" , "Tumbler or flute?" , "Philip --, US composer" , "measure of wine" .)