Return the call (4)
I believe the answer is:
echo
This might be a double definition.
'return' is the first definition.
(I have seen 'Return of sound' mean 'echo' so perhaps 'return' could also mean 'echo')
'the call' is the second definition.
The answer and definition can be both related to communication as well as being singular nouns.
Maybe they are linked in a way I don't understand?
Can you help me to learn more?
(Other definitions for echo that I've seen before include "Imitate -- a nymph" , "Before foxtrot" , "Parrot" , "In The Maid of the Mountains" , "Reflected radar beam" .)