Peer who's almost unprotected (4)
I believe the answer is:
'peer' is the definition.
(earl is a kind of peer)
'almost unprotected' is the wordplay.
'almost' becomes 'nearly' ('nearly' can be a synonym of 'almost').
'unprotected' says to take the centre.
The centre of 'nearly' is 'EARL'.
'who's' is the link.
(Other definitions for earl that I've seen before include "A real titled lord" , "Rank below marquess" , "A real peer" , "Countesss husband" , "Real (anag) - nobleman" .)