Flour source (5)
I believe the answer is:
wheat
'flour source' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for wheat that I've seen before include "Annual grass with light brown grains" , "Cereal grass" , "Grass whose grain is used to make flour" , "Cereal plant" , "Hat we turn into grain" .)