Cave sound (4)
I believe the answer is:
echo
'cave sound' 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 "Reverberate - nymph spurned by Narcissus" , "Sound heard before" , "One who slavishly copies" , "Reverberation of sound" , "Say again or imitate" .)