Celebrity’s scornful expression when upset (4)
I believe the answer is:
star
'celebrity's' is the definition.
(I've seen this before)
'scornful expression when upset' is the wordplay.
'scornful expression' becomes 'rats'.
'when upset' says the letters should be written in reverse (in a down clue - set, written, up).
'rats' reversed gives 'STAR'.
(Other definitions for star that I've seen before include "Polaris, possibly" , "Take the lead role" , "appear above others?" , "Bright" , "''Twinkle, twinkle, little ...'' (children's song)" .)