Root fool dunked in wine (7)
I believe the answer is:
cassava
'root' is the definition.
(edible root)
'fool dunked in wine' is the wordplay.
'fool' becomes 'ass' (both can mean a stupid person).
'dunked in' means one lot of letters goes inside another.
'wine' becomes 'cava' (type of Spanish wine).
'ass' inserted into 'cava' is 'CASSAVA'.
(Other definitions for cassava that I've seen before include "source of starch" , "Edible starchy root" , "starchy food" , "Staple food, the source of tapioca" , "tree" .)