The group to admit religious instruction? (4)
I believe the answer is:
trio
'the group' is the definition.
(trio is a kind of group)
'to admit religious instruction?' is the wordplay.
'admit' indicates putting letters inside.
'religious instruction?' becomes 'ri' (abbreviation).
'to' placed around 'ri' is 'TRIO'.
(Other definitions for trio that I've seen before include "(Composition for) three performers" , "A group or set of three" , "Threesome" , "Work for three" , "Chamber group" .)