Girl is to receive Religious Instruction (4)
I believe the answer is:
iris
'girl' is the definition.
'is to receive religious instruction' is the wordplay.
'to receive' is an insertion indicator.
'religious instruction' becomes 'ri' (abbreviation).
'is' going around 'ri' is 'IRIS'.
(Other definitions for iris that I've seen before include "bit of a looker" , "may open up in camera" , "Flag - part of eye" , "Fleur-de-lis" , "Flag - surrounding pupil" .)