One ultimately knowledgeable in scripture? (3)
I believe the answer is:
rev
'one' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'ultimately knowledgeable in scripture?' is the wordplay.
'ultimately' suggests the final letters.
'in' means one lot of letters goes inside another.
'scripture?' becomes 'rv' (Revised Version, revision of King James Bible).
The last letter of 'knowledgeable' is 'e'.
'e' put inside 'rv' is 'REV'.
(Other definitions for rev that I've seen before include "Speed up engine (colloq.)" , "Gun (an engine)" , "vicar?" , "Cleric" , "Run (engine) quickly" .)