Ray in student group returning smile (7)
I believe the answer is:
sunbeam
'ray' is the definition.
(sunbeam is a kind of ray)
'student group returning smile' is the wordplay.
'student group' becomes 'nus' (National Union of Students).
'returning' shows that the letters should be reversed in order.
'smile' becomes 'beam' (beaming is a kind of smiling).
'nus' in reverse letter order is 'sun'.
'sun'+'beam'='SUNBEAM'
'in' acts as a link.
(Other definitions for sunbeam that I've seen before include "Bright ray -- bus name (anag)" , "Ray of natural light" , "Bright shaft of daylight" , "Ray of solar light" , "sheds a bit of light" .)