One bouncing off the walls ultimately became lethargic couch potato (4)
I believe the answer is:
echo
'one bouncing off the walls' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.
'ultimately became lethargic couch potato' is the wordplay.
'ultimately' suggests the final letters.
The last letters of 'became lethargic couch potato' are 'ECHO'.
Can you help me to learn more?
(Other definitions for echo that I've seen before include "Be suggestive of" , "In The Maid of the Mountains" , "Sound heard before" , "Repeated sound" , "Reverberate - nymph spurned by Narcissus" .)