Nobleman brands serf's head (8)
I believe the answer is:
marquess
'nobleman' is the definition.
(marquess is a kind of nobleman)
'brands serf's head' is the wordplay.
'brands' becomes 'marques' ('marque' can be a synonym of 'brand').
'head' suggests taking the first letters.
The first letter of 'serf' is 's'.
'marques'+'s'='MARQUESS'
(Other definitions for marquess that I've seen before include "He's entitled" , "Lady of nobility" , "noble" , "Nobleman" .)