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 see how they can define each other.
'ultimately became lethargic couch potato' is the wordplay.
'ultimately' indicates one should take the final letters.
The final 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" .)