The yellow of an egg (4)
I believe the answer is:
yolk
'the yellow of an egg' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for yolk that I've seen before include "Soldiers go in here" , "Food kept by white" , "Part of a 7 Down" , "Yellow (as opposed to white?)" , "something attractive to soldiers?" .)