Writer for children allows light into study (6)
I believe the answer is:
crayon
'writer for children' is the definition.
(writing implement which a child might use)
'light into study' is the wordplay.
'light' becomes 'ray' (ray is a kind of light).
'into' means one lot of letters goes inside another.
'study' becomes 'con' (con can informally mean to study).
'ray' inserted inside 'con' is 'CRAYON'.
'allows' is the link.
(Other definitions for crayon that I've seen before include "Wax colouring stick" , "Drawing-stick" , "Coloured wax pencil" , "Colouring pencil" , "Stick to colour" .)