Smart reply (6)
(Other definitions for retort that I've seen before include "may be found in lab" , "A quick, sharp reply" , "Lab. vessel; quip" , "A sharp reply or vessel used in distilling" , "Witty reply" .)
'smart reply' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for riposte that I've seen before include "Rejoinder" , "Sally" , "Snappy comeback" , "Witty retort" , "Reply, response" .)