Lord _ there's a duck in the barn! (5)
I believe the answer is:
baron
'lord' is the definition.
(baron is a kind of lord)
'a duck in the barn' is the wordplay.
'a duck' becomes 'o' (resembles zero - 'duck' in cricket).
'in the' means one lot of letters goes inside another.
'o' inserted inside 'barn' is 'BARON'.
'there's' is the link.
(Other definitions for baron that I've seen before include "Lowest rank of British nobility" , "captain of industry" , "Minor noble" , "Powerful industrialist" , "Minor nobleman" .)