A sum Ben worked out for Ray (7)
I believe the answer is:
sunbeam
'for ray' is the definition.
(sunbeam is a kind of ray)
'a sum ben worked out' is the wordplay.
'worked out' is an anagram indicator.
'a'+'sum'+'ben'='asumben'
'asumben' is an anagram of 'SUNBEAM'.
(Other definitions for sunbeam that I've seen before include "Ray of solar light" , "Bright ray -- bus name (anag)" , "Bright shaft of daylight" , "sheds a bit of light" , "Ray of natural light" .)