This will reflect what the drinker uses (5)
I believe the answer is:
glass
'this will' is the definition.
The definition and answer can be both to do with ownership as well as being verbs in their base form.
Perhaps you can see an association between them that I can't see?
I don't 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 "Greenhouse material" , "Perhaps schooner" , "Transparent substance" , "Transparent stuff" , "Goblet" .)