Sun's against religious education being about you! (6)
I believe the answer is:
solver
'you' is the definition.
(the solver of this crossword puzzle!)
'sun's against religious education being about' is the wordplay.
'sun' becomes 'sol' (Latin for 'sun').
'against' becomes 'v' (versus).
'religious education' becomes 'RE' (abbreviation).
'being about' says the letters should be written backwards.
're' written backwards gives 'er'.
'sol'+'v'+'er'='SOLVER'
(Other definitions for solver that I've seen before include "You find" , "Private Eye" , "One succeeding with crossword" , "One finds the answer to a mystery" , "You're one, for instance" .)