Liver, for one (5)
I believe the answer is:
organ
'liver for one' is the definition.
(I know that liver is a type of internal organ)
This is all the clue.
(Other definitions for organ that I've seen before include "music producer" , "Heart or lung, say" , "Medium; body part" , "Word after "pipe" or "barrel"" , "Church keyboard instrument" .)