Splendid religious instruction by church (4)
I believe the answer is:
rich
'splendid' is the definition.
(thesaurus)
'religious instruction by church' is the wordplay.
'religious instruction' becomes 'ri' (abbreviation).
'by' is a charade indicator (letters next to each other).
'church' becomes 'ch' (common abbreviation**).
'ri'+'ch'='RICH'
(Other definitions for rich that I've seen before include "elaborately decorated" , "Abundant" , "Lord Chancellor" , "Outrageous" , "Highly-flavoured" .)