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