Noble title (5)
(Other definitions for duke that I've seen before include "Highest-ranking nobleman" , "Nobleman - fist" , "Highest ranking British peer" , "Highest hereditary title in Britain" , "noble" .)
(Other definitions for lord that I've seen before include "Proprietor of a manor?" , "10th" , "Master, ruler or titled gent" , "Until 2006 the presiding officer of the House of Lords and head of the judiciary" , "Entitled" .)
'noble title' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for earl that I've seen before include "Sandwich perhaps" , "British noble" , "Rank immediately above viscount" , "House member" , "UK nobleman ranking between marquess and viscount" .)
(Other definitions for lady that I've seen before include "mature woman" , "Female peer" , "See 20" , "Gentlewoman" , "Woman, maybe titled" .)
(Other definitions for baron that I've seen before include "Minor peer" , "An English lord or a magnate" , "Lowest rank of British nobility" , "Minor noble" , "powerful businessman" .)