Of the kidneys (5)
I believe the answer is:
renal
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">of the kidneys</span>' is the definition.<br/ ><span class="explanation-format__explanation">(renal means relating to the kidneys)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for renal that I've seen before include "Referring to organ" , "using the reins" , "Vital organ's" , "Like an organ" , "associated with organ" .)