Spud serves up drink with a measure of acidity for Fourth of July (6)
I believe the answer is:
murphy
'spud' is the definition.
('murphy' can be a synonym of 'spud')
'serves up drink with a measure of acidity for fourth of july' is the wordplay.
'serves up' says the letters should be written in reverse.
'drink' becomes 'rum' (I've seen this before).
'with' is a charade indicator (letters next to each other).
'a measure of acidity for' becomes 'ph' (I am not sure about this - if you are sure you should believe this answer much more).
'fourth of july' becomes 'y' (fourth letter of the word 'July').
'rum' in reverse letter order is 'mur'.
'mur'+'ph'+'y'='MURPHY'
Can you help me to learn more?
(Other definitions for murphy that I've seen before include "King Edward, for example" , "King Edward, say" , "lawgiver?" , "Sod (in terms of a law) - 19" , "Irish potato" .)