Lavish religious instruction supplied by church (4)
I believe the answer is:
rich
'lavish religious instruction' is the definition.
'rich' can be an answer for 'lavish' (thesaurus). I am unsure of the 'religious instruction' bit.
'supplied by church' is the wordplay.
'supplied by' becomes 'ri' (I am not sure about this - if you are sure you should believe this answer much more).
'church' becomes 'ch' (common abbreviation).
'ri'+'ch'='RICH'
Can you help me to learn more?
(Other definitions for rich that I've seen before include "Lord Chancellor" , "Well-heeled" , "(Person) with plenty of money" , "Of great worth or quality" , "having much wealth" .)