Island seized by fat landowner (5)
I believe the answer is:
laird
'landowner' is the definition.
(Scottish term for a landowner)
'island seized by fat' is the wordplay.
'island' becomes 'i'.
'seized by' indicates putting letters inside.
'fat' becomes 'lard' (I've seen this before).
'i' put into 'lard' is 'LAIRD'.
(Other definitions for laird that I've seen before include "Scots landowner" , "Scot. landowner" , "Scottish squire" , "rich Scotsman" , "Scots estate owner" .)