Ray of light (7)
(Other definitions for sunbeam that I've seen before include "Ray of natural light" , "sheds a bit of light" , "Bright shaft of daylight" , "Ray of solar light" , "Bright ray -- bus name (anag)" .)
'ray of light' is the definition.
(I know this)
This is the entire clue.
(Other definitions for beam that I've seen before include "Radiant smile; timber support" , "Shine" , "Supporting structure" , "XWD SETTER" , "Ray of light or a large smile" .)