Landowner one fat in girth (5)
I believe the answer is:
laird
'landowner' is the definition.
(Scottish term for a landowner)
'one fat in girth' is the wordplay.
I cannot really see how this works, but
'one' could be 'i' (Roman numeral) and 'i' is found within the answer.
'fat' could be 'lard' (lard is a kind of fat) and 'lard' is found in the leftover letters.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for laird that I've seen before include "Scottish estate-owner" , "rich Scotsman" , "Man owning land" , "Proprietor" , "Scottish owner of large estate" .)