Repeated (sound) (6)
'repeated sound' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for echo that I've seen before include "Reflection of sound - Greek nymph" , "A natural sound reflection" , "Reverberate - nymph spurned by Narcissus" , "call back" , "'Greek nymph, she pined away until only her voice remained (4)'" .)
(Other definitions for echoed that I've seen before include "copied" , "said nothing original" , "Rang" , "Resounded" , "Reverberated; repeated" .)