Cheerful announcement by disheartened girl (4)
I believe the answer is:
glad
'cheerful' is the definition.
('glad' can be similar in meaning to 'cheerful')
'announcement by disheartened girl' is the wordplay.
'announcement' becomes 'ad'.
'by' is a charade indicator (letters next to each other).
'disheartened' means to remove the middle letters.
'girl' with its centre taken out is 'gl'.
'ad' after 'gl' is 'GLAD'.
(Other definitions for glad that I've seen before include "Informal flower < -girl" , "Little flowering plant" , "Feeling happy" , "Joyful" , "More than willing" .)