Source of protein
I believe the answer is:
meat
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">source of protein</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for meat that I've seen before include "Beef, chicken, pork etc" , "Animal flesh as food" , "Ham, pork, chicken etc" , "Chops, steaks etc" , "Spam, ham, lamb etc" .)