South Seas starch (4)
I believe the answer is:
taro
'south seas starch' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for taro that I've seen before include "Starchy plant" , "Starchy food" , "Chinese eddo" , "A rot turns the lily-like plant" , "Tropical plant with edible tuberous roots" .)