Mum, enthusiastic about religion (6)
I believe the answer is:
shinto
'religion' is the definition.
(Shinto is an example)
'mum enthusiastic' is the wordplay.
'mum' becomes 'sh' (I've seen this in other clues).
'enthusiastic' becomes 'into' (I've seen this before).
'sh'+'into'='SHINTO'
'about' is the link.
(Other definitions for shinto that I've seen before include "Japanese worship" , "faith" , "Japanese religion revering ancestors" , "Japenese religion revering ancestors" , "Religious system" .)