You've heard it before (4)
I believe the answer is:
echo
'you've heard it before' 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 "'Greek nymph, she pined away until only her voice remained (4)'" , "next to delta" , "In The Maid of the Mountains" , "The distant sound" , "Reverberate - nymph spurned by Narcissus" .)