Rigidly precise, restricting small source of coloured light (5)
I believe the answer is:
prism
'source of coloured light' is the definition.
(light may be refracted through a prism)
'rigidly precise restricting small' is the wordplay.
'rigidly precise' becomes 'prim' (prim can mean precise or proper).
'restricting' indicates putting letters inside.
'small' becomes 's' (abbreviation - e.g. clothes size).
'prim' enclosing 's' is 'PRISM'.
(Other definitions for prism that I've seen before include "Theatrical Miss" , "Light beam splitter" , "Transparent block" , "What's on Dark Side of the Moon" , "Prims (anag)" .)