One welcomed by fat landowner (5)
I believe the answer is:
laird
'landowner' is the definition.
(Scottish term for a landowner)
'one welcomed by fat' is the wordplay.
'one' becomes 'i' (Roman numeral).
'welcomed by' means one lot of letters goes inside another (letters welcomed inside).
'fat' becomes 'lard' (lard is a kind of fat).
'i' inserted into 'lard' is 'LAIRD'.
(Other definitions for laird that I've seen before include "Scottish estate-owner" , "Scottish squire" , "Scottish landowner" , "Scottish owner of large estate" , "He's landed in Scotland" .)