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 informally mean to study).
'ray' inserted within 'con' is 'CRAYON'.
'reveals' is the 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" .)