Sauce bean (4)
I believe the answer is:
soya
'sauce bean' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for soya that I've seen before include "Plant grown for its beans" , "Bean used in dark sauce" , "Asian bean yielding edible oil and flour" , "Protein-rich bean" , "Plant producing beans" .)