Tapioca source (7)
I believe the answer is:
'tapioca source' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for cassava that I've seen before include "Manioc, tapioca root" , "starchy food" , "Tropical food" , "Staple food, the source of tapioca" , "source of starch" .)