Source of starch fool put in Spanish fizz (7)
I believe the answer is:
cassava
'source of starch' is the definition.
(I've seen this before)
'fool put in spanish fizz' is the wordplay.
'fool' becomes 'ass' (ass is a kind of fool).
'put in' means one lot of letters goes inside another.
'spanish fizz' becomes 'cava' (I've seen this before).
'ass' going within 'cava' is 'CASSAVA'.
(Other definitions for cassava that I've seen before include "Tropical food" , "Root crop of the tropics" , "starchy food" , "tree" , "Tropical American starchy plant" .)