You might respond to it by eliminating waste (4,2,6)
I believe the answer is:
call of nature
'eliminating waste' is the definition.
I know nothing about this answer so I cannot judge whether this works.
'you might respond to it by' is the wordplay.
I cannot really understand how this works, but
'you' could be 'u' (commonly used in online communication) and 'u' is found within the answer.
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is found within the answer.
'by' could be 'of' and 'of' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for call of nature that I've seen before include "Need to go" , "low? - with 21a" , "Urge to spend a penny" .)