Island seized by fat Scottish landowner (5)
I believe the answer is:
laird
'scottish landowner' is the definition.
(I've seen this before)
'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' going within '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" .)