Organ donor has left for good (5)
I believe the answer is:
liver
'organ' is the definition.
(liver is a kind of organ)
'donor has left for good' is the wordplay.
I cannot quite understand how this works, but
'left' could be 'l' (common abbreviation) and 'l' is found within the answer.
The remaining letters 'iver' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for liver that I've seen before include "Human organ" , "Internal organ" , "Large organ of body" , "Large organ in the abdomen secreting bile" , "Organ; reddish-brown" .)