more cheerful (7)
'more cheerful' is the definition.
('happy' can be similar in meaning to 'cheerful')
This is all the clue.
(Other definitions for happier that I've seen before include "Gladder" , "More joyful" , "More cheerful" .)
'more cheerful' is the definition.
('sunny' can be similar in meaning to 'cheerful')
This is the entire clue.
(Another definition for sunnier that I've seen is " Better weather".)