Snappy comeback (6)
I believe the answer is:
retort
'snappy comeback' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for retort that I've seen before include "Quick and clever or angry reply" , "one used in lab" , "Sharp reply - lab vessel" , "Make quick or angry reply" , "Witty answer" .)