Thyroid, eg (5)
I believe the answer is:
gland
'thyroid eg' is the definition.
(thyroid is a kind of gland)
This is the entire clue.
(Other definitions for gland that I've seen before include "it could be described as secretive" , "liver, for example" , "Organ that releases essential substances into the bloodstream" , "vital structure" , "Organ secreting chemical substances" .)