On reflection, curses celebrity (4)
I believe the answer is:
star
'celebrity' is the definition.
(a star is a celebrity or famous person)
'on reflection curses' is the wordplay.
'on reflection' says the letters should be written in reverse.
'curses' becomes 'rats' (both are mild oaths or expressions of annoyance).
'rats' reversed gives 'STAR'.
(Other definitions for star that I've seen before include "Famous performer" , "A .... called Henry (Roddy Doyle)" , "Top celebrity" , "E.g.Sirius" , "virtuoso" .)