Snappy reply (7)
'snappy reply' 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 rejoinder" , "Still" , "Part of chemist's equipment" , "Make quick or angry reply" , "Sharp response; lab item" .)
(Other definitions for express that I've seen before include "Utter; speedy" , "Utter - direct" , "Train; utter" , "Quick -- exact" , "Verbalise - very fast" .)