Holds water to swallow / drink (6,2)
I believe the answer is:
stands up
'holds water' is the definition.
The answer and definition can be both to do with fundamental existence as well as being verbs in their -s form.
Maybe you can see a link between them that I don't see?
'swallow drink' is the wordplay.
'swallow' becomes 'stand' (I can't explain this - if you can you should believe this answer much more).
'drink' becomes 'sup' (to sup is to drink).
'stand'+'sup'='STANDS UP'
'to' is the link.
Can you help me to learn more?
(Other definitions for stands up that I've seen before include "fails to meet" , "Rises; bears examination" , "Ceases to sit, adopts vertical posture" , "Reacts to 15 [NATIONAL ANTHEM]" , "Rises; bears investigation" .)