Student of the sun's written about carbon and hydrogen (7)
I believe the answer is:
scholar
'student' is the definition.
(synonyms)
'the sun's written about carbon and hydrogen' is the wordplay.
'the sun's' becomes 'solar' (I've seen this before).
'written about' means one lot of letters goes inside another.
'carbon' becomes 'c' (chemical symbol).
'and' is a charade indicator (letters next to each other).
'hydrogen' becomes 'h' (chemical symbol).
'c'+'h'='ch'
'solar' enclosing 'ch' is 'SCHOLAR'.
'of' is the link.
(Other definitions for scholar that I've seen before include "Student or learned person" , "Academic; pupil" , "Noted academic" , "Pupil - erudite person" , "Subject of poem by 1 across" .)