One is welcomed by fat landowner (5)
I believe the answer is:
laird
'landowner' is the definition.
(Scottish term for a landowner)
'one is welcomed by fat' is the wordplay.
'one' becomes 'i' (Roman numeral).
'is welcomed by' indicates putting letters inside (I've seen this in other clues).
'fat' becomes 'lard' (lard is a kind of fat).
'i' placed inside 'lard' is 'LAIRD'.
(Other definitions for laird that I've seen before include "Scottish owner of large estate" , "Scots estate owner" , "Scots landowner" , "Big landowner" , "Scottish squire" .)