Religious instruction's wearing to a small group (4)
I believe the answer is:
trio
'a small group' is the definition.
(I've seen this before)
'religious instruction's wearing to' is the wordplay.
'religious instruction' becomes 'ri' (abbreviation).
'wearing' is an insertion indicator.
'ri' placed into 'to' is 'TRIO'.
(Other definitions for trio that I've seen before include "Beethoven's 'Archduke', for example" , "Bee Gees, Supremes or Andrews Sisters, say" , "Group of three (musicians)" , "Archduke, possibly" , "Musical group" .)