Island seized by fat Scottish landowner (5)
I believe the answer is:
laird
'scottish landowner' is the definition.
(I've seen this in another clue)
'island seized by fat' is the wordplay.
'island' becomes 'i' (geographical abbreviation).
'seized by' is an insertion indicator.
'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 "He's landed in Scotland" , "Scottish owner of large estate" , "rich Scotsman" , "Scot. landowner" , "Big landowner" .)