Island patrolled by fat northern landowner (5)
I believe the answer is:
laird
'landowner' is the definition.
(Scottish term for a landowner)
'island patrolled by fat northern' is the wordplay.
I cannot really understand how this works, but
'island' could be 'i' (geographical abbreviation) and 'i' is located in the answer.
'fat' could be 'lard' (lard is a kind of fat) and 'lard' is present in the remaining letters.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for laird that I've seen before include "Scots landowner" , "Scottish landowner" , "Scottish estate-owner" , "rich Scotsman" , "Proprietor" .)