Identical response (4)
I believe the answer is:
echo
'identical response' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for echo that I've seen before include "next to delta" , "Reflect sound" , "Audio effect" , "Let's hear it again for the Greek nymph who loved Narcissus in vain" , "copy" .)