Get rid of old joke, perhaps, for a comeback? (7)
I believe the answer is:
expunge
'get rid of' is the definition.
'old joke perhaps for a comeback?' is the wordplay.
'old' becomes 'ex' (prefix meaning former or onetime).
'joke' becomes 'pun' (punning is a kind of joking).
'perhaps' becomes 'eg' (for example).
'for a comeback?' is a reversal indicator.
'eg' backwards is 'ge'.
'ex'+'pun'+'ge'='EXPUNGE'
(Other definitions for expunge that I've seen before include "Scrub out" , "Efface" , "Eradicate" , "Completely remove" , "Remove (from records)" .)