Thus penning clue for religion (6)
I believe the answer is:
shinto
'religion' is the definition.
(Shinto is an example)
'thus penning clue' is the wordplay.
'thus' becomes 'so' ('so' can be a synonym of 'thus').
'penning' is an insertion indicator.
'clue' becomes 'hint' ('hint' can be a synonym of 'clue').
'so' placed around 'hint' is 'SHINTO'.
'for' acts as a link.
(Other definitions for shinto that I've seen before include "Japanese cult" , "Japanese religion revering ancestors" , "State religion of Japan until 1945" , "Japanese worship" , "Japenese religion revering ancestors" .)