Landowner in den with duke (5)
I believe the answer is:
laird
'landowner' is the definition.
(Scottish term for a landowner)
'den with duke' is the wordplay.
'den' becomes 'lair' ('lair' can be a synonym of 'den').
'with' means one lot of letters go next to another.
'duke' becomes 'd'.
'lair'+'d'='LAIRD'
'in' is the link.
(Other definitions for laird that I've seen before include "Scottish estate-owner" , "Big landowner" , "Proprietor" , "Scot. landowner" , "Estate landowner" .)