Urinary organ (7)
I believe the answer is:
bladder
'urinary organ' is the definition.
(I know that bladder is a type of urinary organ)
This is the entire clue.
(Other definitions for bladder that I've seen before include "organ" , "Bag in the lower abdomen" , "Distensible sac" , "Chamber" , "Bag containing liquid or gas" .)