Mild expletive upset celebrity (4)
I believe the answer is:
star
'celebrity' is the definition.
(a star is a celebrity or famous person)
'mild expletive upset' is the wordplay.
'mild expletive' becomes 'rats' (I've seen this in another clue).
'upset' is a reversal indicator.
'rats' in reverse letter order is 'STAR'.
(Other definitions for star that I've seen before include "A .... called Henry (Roddy Doyle)" , "Eg, Rigel, Aldebaran" , "Outstanding actor" , "Famous performer" , "Red Dwarf?" .)