The idea of an ancient barbarian going to church! (5)
I believe the answer is:
hunch
'the idea' is the definition.
(hunch is a kind of idea)
'an ancient barbarian going to church' is the wordplay.
'an ancient barbarian' becomes 'hun' (as in Atilla the Hun. I am not sure about the 'ancient' bit.).
'going to' is a charade indicator (letters next to each other).
'church' becomes 'ch' (common abbreviation).
'hun'+'ch'='HUNCH'
'of' acts as a link.
Can you help me to learn more?
(Other definitions for hunch that I've seen before include "Intuitive guess" , "Guess -- sit with bad posture" , "Premonition; hump" , "Vague suspicion" , "Inkling" .)