Alpine answer (4)
I believe the answer is:
echo
'alpine answer' 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 "letter we hear" , "Nymph; E (radio comms.)" , "'Greek nymph, she pined away until only her voice remained (4)'" , "Nymph (only able to repeat the last words of others)" , "Sound heard before" .)