Yellow part of an egg (4)
I believe the answer is:
yolk
'yellow part of an egg' is the definition.
(I've seen this before)
This is the entire clue.
(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"" .)