Second husband knowledgeable about Japanese religion (6)
I believe the answer is:
shinto
'japanese religion' is the definition.
(I've seen this before)
'second husband knowledgeable about' is the wordplay.
'second' becomes 's' (common abbreviation).
'husband' becomes 'h'.
'knowledgeable about' becomes 'into' (I've seen this before).
's'+'h'+'into'='SHINTO'
(Other definitions for shinto that I've seen before include "faith" , "Japanese cult" , "Japanese ancestor-worshipping religion" , "Japenese religion revering ancestors" , "Religious system" .)