Naturalist may study this 22 23 at end of a sentence (4,4)
I believe the answer is:
life form
'naturalist may study this' is the definition.
'22 23 at end of a sentence' is the wordplay.
'oo or' becomes 'form' (I can't explain this - if you can you should believe this answer much more).
'at end of' is a charade indicator (letters next to each other).
'a sentence' becomes 'life' (I have seen 'Longest sentence' mean 'life' so perhaps 'sentence' could also mean 'life').
'form' put after 'life' is 'LIFE FORM'.
(Another definition for life form that I've seen is " Being".)