Yellow part of an egg (4)
I believe the answer is:
yolk
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">yellow part of an egg</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 yolk that I've seen before include "Part of a 7 Down" , "Soldiers go in here" , "Bird food" , "Yellow (as opposed to white?)" , "Egg's "yellow"" .)