Famous entertainer deserts, upset (4)
I believe the answer is:
star
'famous entertainer' is the definition.
(I've seen this before)
'deserts upset' is the wordplay.
'deserts' becomes 'rats' (ratting is a kind of deserting).
'upset' is a reversal indicator.
'rats' backwards is 'STAR'.
(Other definitions for star that I've seen before include "Arts (anag)" , "Red Dwarf?" , "Celestial body of hot gases" , "Outstanding entertainer" , "Celebrated performer" .)