Island included by fat squire (5)
I believe the answer is:
laird
'squire' is the definition.
(thesaurus)
'island included by fat' is the wordplay.
'island' becomes 'i' (geographical abbreviation).
'included by' indicates putting letters inside.
'fat' becomes 'lard' (lard is a kind of fat).
'i' put within 'lard' is 'LAIRD'.
(Other definitions for laird that I've seen before include "Estate landowner" , "Scots estate owner" , "Scottish landowner" , "Proprietor" , "He's landed in Scotland" .)