Stupid Brown attending church (5)
I believe the answer is:
dunce
'stupid' is the definition.
(I know that dunce is a type of stupid)
'brown attending church' is the wordplay.
'brown' becomes 'dun' (dun is a kind of brown).
'attending' is a charade indicator (letters next to each other).
'church' becomes 'ce' (abbreviation for Church of England).
'dun'+'ce'='DUNCE'
(Other definitions for dunce that I've seen before include "Old word for person slow at learning" , "An ignoramus" , "Simpleton" , "One slow to pick" , "School fool" .)