Aristocrat round in farm building (5)
I believe the answer is:
baron
'aristocrat' is the definition.
(baron is a kind of aristocrat)
'round in farm building' is the wordplay.
'round' becomes 'o' (round can mean a round object).
'in' indicates putting letters inside.
'farm building' becomes 'barn' (barn is a kind of farm building).
'o' put within 'barn' is 'BARON'.
(Other definitions for baron that I've seen before include "Title of rank" , "An English lord or a magnate" , "powerful businessman" , "captain of industry" , "Lowest rank of British peerage" .)