One taken in by a fat Scottish landowner (5)
I believe the answer is:
laird
'scottish landowner' is the definition.
(I've seen this before)
'one taken in by a fat' is the wordplay.
'one' becomes 'i' (Roman numeral).
'taken in by' indicates putting letters inside.
'a fat' becomes 'lard' (lard is a kind of fat).
'i' going inside 'lard' is 'LAIRD'.
(Other definitions for laird that I've seen before include "Big landowner" , "He's landed in Scotland" , "Scottish owner of large estate" , "rich Scotsman" , "Scots estate owner" .)