Wally's quick rejoinder partly backfired (4)
I believe the answer is:
jerk
'wally's' is the definition.
(both can mean a stupid person)
'quick rejoinder partly backfired' is the wordplay.
'partly' indicates a hidden word (part of the clue is used).
'backfired' is a reversal indicator.
'krej' is hidden within 'quick rejoinder'.
'krej' backwards is 'JERK'.
(Other definitions for jerk that I've seen before include "Yank, idiot" , "Abrupt movement" , "Jolt; tug" , "Sudden tug" , "kind of lift in competition" .)