Collar found in Dowton Abbey (3)
I believe the answer is:
nab
'collar' is the definition.
(both can mean to catch a person)
'in dowton abbey' is the wordplay.
'in' says the answer is hidden in the clue.
'NAB' is hidden within 'dowton abbey'.
'found' acts as a link.
(Other definitions for nab that I've seen before include "Run in" , "nail" , "Collar (sl.)" , "Apprehend" , "Catch (thief) (colloq.)" .)