Organ that secretes (5)
I believe the answer is:
gland
'organ that secretes' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for gland that I've seen before include "Organ that separates substances from blood" , "liver, for example" , "Organ that releases essential substances into the bloodstream" , "vital structure" , "Organ that secretes" .)