Drop to drink (4)
I believe the answer is:
sink
This clue is a double definition.
'drop' is the first definition.
(synonyms)
'drink' is the second definition.
(I've seen this before)
'to' is the link.
(Other definitions for sink that I've seen before include "Drop N flag" , "Subside" , "drink" , "Drop in the ocean?" , "place of corruption" .)