One's beset by fat landowner (5)
I believe the answer is:
laird
'landowner' is the definition.
(Scottish term for a landowner)
'one's beset by fat' is the wordplay.
'one' becomes 'i' (Roman numeral).
'beset by' is an insertion indicator ('beset' could historically mean 'cover').
'fat' becomes 'lard' (I've seen this before).
'i' going inside 'lard' is 'LAIRD'.
(Other definitions for laird that I've seen before include "Scottish owner of large estate" , "Scottish estate-owner" , "He's landed in Scotland" , "Big landowner" , "Estate landowner" .)