Stag (4)
(Other definitions for deer that I've seen before include "does for example" , "See 23" , "maybe does" , "Ruminant mammal with antlers and hooves" , "Wild animal" .)
'stag' is the definition.
(both can mean a male deer)
This is the entire clue.
(Other definitions for hart that I've seen before include "Male red-deer" , "Songwriting partnership, Rodgers and --" , "Richard Rodgers and Lorenz . . . . were famous musical partnership" , "Adult male red deer" , "Stag" .)