Landowner makes den on top of Dartmoor (5)
I believe the answer is:
laird
'landowner' is the definition.
(Scottish term for a landowner)
'den on top of dartmoor' is the wordplay.
'den' becomes 'lair' (synonyms).
'on top of' indicates taking the first letters.
The first letter of 'dartmoor' is 'd'.
'lair'+'d'='LAIRD'
'makes' acts as a link.
(Other definitions for laird that I've seen before include "Proprietor" , "Scot. landowner" , "Man owning land" , "Scots landowner" , "Estate landowner" .)