Soup label mysterious writer keeps (6)
I believe the answer is:
potage
'soup' is the definition.
(potage is a kind of soup)
'label mysterious writer keeps' is the wordplay.
'label' becomes 'tag' (synonyms).
'mysterious writer' becomes 'Poe' (Edgar Allan Poe).
'keeps' means one lot of letters goes inside another (inserted letters are kept inside).
'tag' placed into 'poe' is 'POTAGE'.
(Other definitions for potage that I've seen before include "which is fairly thick" , "> sort of liquid nourishment" , "Stock food item" , "this spoon-fed?" , "Thick, often creamy soup" .)