Relating to the kidneys (5)
I believe the answer is:
renal
'relating to the kidneys' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for renal that I've seen before include "Referring to organ" , "Pertaining to the kidney" , "Concerning the kidneys" , "Concerned with organ" , "like kidneys" .)