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