Yellow of an egg (4)
I believe the answer is:
yolk
'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 "Part of a 7 Down" , "Food kept by white" , "Yellow (as opposed to white?)" , "something attractive to soldiers?" , "Yellow part of an egg" .)