One soon joining chapter? (5)
I believe the answer is:
canon
'one' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'soon joining chapter?' is the wordplay.
'soon' becomes 'anon' (I've seen this before).
'joining' means one lot of letters go next to another.
'chapter?' becomes 'c'.
'anon' after 'c' is 'CANON'.
(Other definitions for canon that I've seen before include "Basic law or principle" , "Moron" , "in the ministry" , "Round, general rule" , "priest for cathedral duties" .)