Snappy reply from a rotter? (6)
I believe the answer is:
retort
'snappy reply' is the definition.
(I've seen this before)
'from a rotter?' is the wordplay.
'from' indicates anagramming the letters (from the letters).
'rotter' with letters rearranged gives 'RETORT'.
(Other definitions for retort that I've seen before include "Long-necked glass vessel" , "one used in lab" , "A quick, sharp reply" , "Quick rejoinder" , "Chem. flask; sharp answer" .)