Proper politician put up to defend religious instruction (4)
I believe the answer is:
prim
'proper' is the definition.
(similar in meaning)
'politician put up to defend religious instruction' is the wordplay.
'politician' becomes 'MP' (Member of Parliament).
'put up' says the letters should be written in reverse.
'to defend' is an insertion indicator.
'religious instruction' becomes 'RI' (abbreviation).
'mp' reversed gives 'pm'.
'pm' going around 'ri' is 'PRIM'.
(Other definitions for prim that I've seen before include "Prudish, proper" , "Prudish, formal" , "Proper (type of rose?)" , "Stiffly correct" , "Straitlaced and proper" .)