Earth dead? Cheer up (5)
I believe the answer is:
elate
'cheer up' is the definition.
(I've seen this in another clue)
'earth dead?' is the wordplay.
'earth' becomes 'E'.
'dead?' becomes 'late' (similar in meaning).
'e'+'late'='ELATE'
(Other definitions for elate that I've seen before include "cheer" , "Encourage" , "Fill with joy" , "Bring joy" , "Overjoy" .)