Light in study reveals source of colour (6)
I believe the answer is:
crayon
'source of colour' is the definition.
(I've seen this before)
'light in study' is the wordplay.
'light' becomes 'ray' (ray is a kind of light).
'in' is an insertion indicator.
'study' becomes 'con' (con can mean to study).
'ray' placed within 'con' is 'CRAYON'.
'reveals' acts as a link.
(Other definitions for crayon that I've seen before include "Child's colouring stick" , "Producer of sketch" , "Wax colouring stick" , "Colouring pencil" , "Drawing implement" .)