A quick reply by one who is foiled (7)
I believe the answer is:
riposte
'a quick reply' is the definition.
(I've seen this before)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for riposte that I've seen before include "Witty retort" , "Quick retort or return hit in fencing" , "Sally" , "Quick and witty reply" , "Quick retort or return thrust in fencing" .)