Nobleman ahead of time? Not unknown (4)
I believe the answer is:
earl
'nobleman' is the definition.
(earl is a kind of nobleman)
'ahead of time? not unknown' is the wordplay.
'ahead of time?' becomes 'early' ('early' can be a synonym of 'ahead of time').
'not' suggests deleting specific letters.
'unknown' becomes 'y' (algebra).
'early' with 'y' taken away is 'EARL'.
(Other definitions for earl that I've seen before include "Rank below marquess" , "British noble" , "Titled man" , "Peer (Br.)" , "Countesss husband" .)