Protein bean (4)
I believe the answer is:
soya
'protein bean' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for soya that I've seen before include "Bean used to make tofu" , "Protein-rich bean" , "I'm rich in oil" , "Asian bean yielding edible oil and flour" , "Type of bean" .)