Maybe The Sun 's making a comeback? I'm not happy about that (4)
I believe the answer is:
rats
'i'm not happy about that' is the definition.
('rats' is an expression of irritation)
'maybe the sun 's making a comeback?' is the wordplay.
'maybe the sun ' becomes 'star' (the sun is a star).
'making a comeback?' says the letters should be written backwards.
'star' backwards is 'RATS'.
(Other definitions for rats that I've seen before include "Oh no" , "Blast" , "were piped away" , "Snoopy's imprecation - vermin" , "Rodent pests" .)