Altogether it's sour, but one drop is sweet (4)
I believe the answer is:
acid
'sweet' is the definition.
Both the definition and answer are adjectives. Perhaps you can see an association between them that I don't see?
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for acid that I've seen before include "Sour - type of rain or drop" , "Alkali's opposite" , "Corrosive substance" , "Bitter - drug" , "Morose" .)