Cheery (4)
'cheery' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for glad that I've seen before include "Feeling pleasure" , "Little flowering plant" , "Made up" , "Happy, pleased" , "bright?" .)
(Other definitions for mirthful that I've seen before include "Merry and laughing" , "Exuding merriment" , "amusing" , "Back-slapping" , "Hilarious" .)