Quickly pass on insult (4,4)
I believe the answer is:
drop dead
I believe this clue is a double definition.
'quickly pass on' is the first definition.
(to drop dead is to pass on quickly or suddenly)
'insult' is the second definition.
('drop dead' may be said to someone as an insult)
(Other definitions for drop dead that I've seen before include "Really offensive remark" , "Way to go!" , "Suddenly fail" , "Absolutely (gorgeous)" , "to appear so gorgeous" .)