Mafia boss found in river (3)
I believe the answer is:
don
I believe this clue is a double definition.
'mafia boss' is the first definition.
(term for a mafia leader)
'river' is the second definition.
(Yorkshire river)
'found in' acts as a link.
(Other definitions for don that I've seen before include "Put on; man's name" , "Put on; college teacher" , "University teacher" , "- - Giovanni, 26" , "Get into" .)