Fist of a nobleman (4)
I believe the answer is:
duke
I believe this clue is a double definition.
'fist' is the first definition.
(I've seen this before)
'nobleman' is the second definition.
(duke is a kind of nobleman)
'of a' is the link.
(Other definitions for duke that I've seen before include "noble" , "Nobleman - fist" , "Highest-ranking nobleman" , "Highest hereditary title in Britain" , "- - Ellington (jazz musician)" .)