Proper to shield singular source of spectral effects (5)
I believe the answer is:
prism
'source of spectral effects' is the definition.
(a prism refracts the spectrum of light)
'proper to shield singular' is the wordplay.
'proper' becomes 'prim' (prim and proper).
'to shield' indicates putting letters inside (some letters protect or shield others).
'singular' becomes 's' (abbreviation sometimes used in dictionaries).
'prim' placed around 's' is 'PRISM'.
(Other definitions for prism that I've seen before include "Light splitting optical device" , "Worthing nurse" , "Light transmitter" , "Transparent solid for dispersing light" , "Light-splitting object" .)