Landowner died, to be buried under den (5)
I believe the answer is:
laird
'landowner' is the definition.
(Scottish term for a landowner)
'died to be buried under den' is the wordplay.
'died' becomes 'd' (abbreviation next to the year of someone's death).
'to be buried under' says to put letters next to each other (in a down clue, some letters go underneath others).
'den' becomes 'lair' (synonyms).
'd' put after 'lair' is 'LAIRD'.
(Other definitions for laird that I've seen before include "Scots landowner" , "Big landowner" , "Scottish estate-owner" , "Scottish squire" , "Scottish landowner" .)