One grabbed by fat landlord (5)
I believe the answer is:
laird
'landlord' is the definition.
(thesaurus)
'one grabbed by fat' is the wordplay.
'one' becomes 'i' (Roman numeral).
'grabbed by' means one lot of letters goes inside another.
'fat' becomes 'lard' (I've seen this before).
'i' placed within 'lard' is 'LAIRD'.
(Other definitions for laird that I've seen before include "Scottish estate-owner" , "Scot. landowner" , "rich Scotsman" , "Scottish landowner" , "Scots estate owner" .)