Existence sucks on the whole (8)
I believe the answer is:
entirety
'the whole' is the definition.
(I've seen this before)
'existence sucks on' is the wordplay.
I cannot really understand how this works, but
'on' could be 're' ('on' can be mean 'with reference to') and 're' is present in the answer.
'existence' could be 'entity' (I've seen this before) and 'entity' is found in the remaining letters.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for entirety that I've seen before include "Absoluteness" , "Whole" , "Sum" , "Totality" , "The lot" .)