Landowner has a den that holds five hundred (5)
I believe the answer is:
laird
'landowner' is the definition.
(Scottish term for a landowner)
'a den that holds five hundred' is the wordplay.
'a den' becomes 'lair' (synonyms).
'that holds' is a charade indicator (letters next to each other).
'five hundred' becomes 'd' (Roman numeral for 500).
'lair'+'d'='LAIRD'
'has' acts as a link.
(Other definitions for laird that I've seen before include "Scots estate owner" , "Scottish squire" , "Scots landowner" , "rich Scotsman" , "He's landed in Scotland" .)