Cook loyally holds back part of egg (4)
I believe the answer is:
yolk
'part of egg' is the definition.
('yolk' is part of 'egg')
'cook loyally holds back' is the wordplay.
'holds' says the answer is hidden in the clue.
'back' says the letters should be written backwards.
'kloy' is hidden in the letters of 'cook loyally'.
'kloy' written backwards gives 'YOLK'.
(Other definitions for yolk that I've seen before include "Bird food" , "Yellow part of an egg" , "Part of a 7 Down" , "Food kept by white" , "Soldiers go in here" .)