About to leave the whole thing (6)
I believe the answer is:
entity
'about to leave the whole thing' is the definition.
I can't tell whether this defines the answer.
'about to leave the whole thing' is the wordplay.
'about' becomes 're' ('re' means regarding or about).
'to leave' indicates named letters should be taken away.
'the whole thing' becomes 'entirety'.
'entirety' with 're' removed is 'ENTITY'.
(Other definitions for entity that I've seen before include "A thing with distinct and independent existence" , "Something with independent existence" , "It exists as a seperate thing (6)" , "Essence, existence" , "Something existing in its own right" .)