Wipe out old joke by German sweetheart? (7)
I believe the answer is:
expunge
'wipe out' is the definition.
(I've seen this in another clue)
'old joke by german sweetheart?' is the wordplay.
'old' becomes 'ex' (prefix meaning former or onetime).
'joke' becomes 'pun' (pun is a type of joke).
'by' says to put letters next to each other.
'german' becomes 'g' (abbreviation).
'sweetheart?' becomes 'e' (the heart/centre of 'sweet').
'ex'+'pun'+'g'+'e'='EXPUNGE'
(Other definitions for expunge that I've seen before include "Cut it out!" , "Scrub out" , "Completely obliterate" , "Delete from the record" , "Completely remove" .)