Knowledgeable about Shintoism? Not entirely (4)
I believe the answer is:
into
'knowledgeable about' is the definition.
(I've seen this before)
'shintoism? not entirely' is the wordplay.
'not entirely' indicates the answer is hidden within the clue.
'INTO' can be found hidden inside 'shintoism'.
(Other definitions for into that I've seen before include "Liking" , "ignition regularly missing" , "Fascinated by" , "A fan of" , "Knowledgeable about" .)