Make cheerful (7)
'make cheerful' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for hearten that I've seen before include "giving cheer" , "Comfort" , "Make (someone) more confident" , "Make cheerful" , "Be encouraged" .)
(Other definitions for exhilarate that I've seen before include "Thrill" , "Cheer" , "Make very happy or very animated" , "Enliven" .)