Celebrity ultimately forgives returning traitor (4)
I believe the answer is:
star
'celebrity' is the definition.
(I've seen this before)
'ultimately forgives returning traitor' is the wordplay.
'ultimately' indicates one should take the final letters.
'returning' shows that the letters should be reversed in order.
'traitor' becomes 'rat' (I've seen this before).
The final letter of 'forgives' is 's'.
'rat' back-to-front is 'tar'.
's'+'tar'='STAR'
(Other definitions for star that I've seen before include "Point of light in the heavens" , "The leading player is heavenly" , "Symbol of excellence" , "Eg, Antares" , "Pointed shape" .)