Storm about backing foreign plonk, sour stuff (7)
I believe the answer is:
vinegar
'sour stuff' is the definition.
(I've seen this before)
'storm about backing foreign plonk' is the wordplay.
'storm' becomes 'rage' ('rage' can be a synonym of 'storm').
'about' says the letters should be written backwards.
'backing' means one lot of letters go next to another.
'foreign plonk' becomes 'vin' (I am not sure about this - if you are sure you should believe this answer much more).
'rage' written backwards gives 'egar'.
'egar' put after 'vin' is 'VINEGAR'.
Can you help me to learn more?
(Other definitions for vinegar that I've seen before include "Pickling medium" , "Sourish liquid for flavouring chips" , "bad wine?" , "Pickling agent" , "Soured wine, cider etc." .)