Peer left, behind organ (4)
I believe the answer is:
earl
'peer' is the definition.
(I know that earl is a type of peer)
'left behind organ' is the wordplay.
'left' becomes 'l' (common abbreviation).
'behind' means one lot of letters go next to another.
'organ' becomes 'ear' (the ear is an organ).
'l' put after 'ear' is 'EARL'.
(Other definitions for earl that I've seen before include "Real kind of English lord" , "Aristocrat ranking between marquis and viscount" , "A real peer" , "British nobleman" , "Real (anag) - nobleman" .)