Prayer of oneness: “When one one leaves, another one enters” (8)
I believe the answer is:
entreaty
'prayer' is the definition.
(synonyms)
'oneness when one one leaves another one enters' is the wordplay.
I cannot quite understand how this works, but
'when' could be 'e' (I've seen this in other clues) and 'e' is found in the answer.
'one' could be 'a' and 'a' is found in the answer.
'another' could be 're' (I've seen this in other clues) and 're' is present in the answer.
This may be the basis of clue (or it may be nonsense).
'of' acts as a link.
(Other definitions for entreaty that I've seen before include "Earnest or urgent request" , "application for assistance" , "Earnest plea" , "petition" , "Teeny rat (anag.)" .)