Nobleman seen in rear light (4)
I believe the answer is:
earl
'nobleman' is the definition.
(member of the nobility)
'in rear light' is the wordplay.
'in' indicates a hidden word.
'EARL' is hidden within 'rear light'.
'seen' acts as a link.
(Other definitions for earl that I've seen before include "Sandwich perhaps" , "A real peer" , "Real (anag) - nobleman" , "Peer of the realm" , "Rank below marquess" .)