Tofu source (4)
I believe the answer is:
soya
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">tofu source</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for soya that I've seen before include "Plant seed, meat substitute" , "source of protein" , "Bean used in dark sauce" , "Animal-substitute (bean)" , "Type of bean" .)