Existence is about relating to everything (8)
I believe the answer is:
entirety
'everything' is the definition.
(thesaurus)
'existence is about relating to' is the wordplay.
'existence' becomes 'entity' (I've seen this in another clue).
'is about' indicates putting letters inside.
'relating to' becomes 're' (re can mean with regards to).
'entity' enclosing 're' is 'ENTIRETY'.
(Other definitions for entirety that I've seen before include "Totality" , "Sum" , "Completeness" , "The lot" , "Absoluteness" .)