Fullness of being, about to be limited (8)
I believe the answer is:
entirety
'fullness' is the definition.
(thesaurus)
'being about to be limited' is the wordplay.
'being' becomes 'entity' (I've seen this in other clues).
'about' is an insertion indicator.
'to be limited' becomes 're' (I can't justify this - if you can you should give a lot more credence to this answer).
'entity' placed around 're' is 'ENTIRETY'.
'of' is the link.
Can you help me to learn more?
(Other definitions for entirety that I've seen before include "Whole" , "The lot" , "Universe" , "Totality" , "Sum" .)