Believer embraces alternative thinker (8)
I believe the answer is:
theorist
'thinker' is the definition.
(I've seen this in another clue)
'believer embraces alternative' is the wordplay.
'believer' becomes 'theist' (someone who believes in a god).
'embraces' means one lot of letters goes inside another.
'alternative' becomes 'or'.
'theist' enclosing 'or' is 'THEORIST'.
(Other definitions for theorist that I've seen before include "Speculator" , "Creator of hypothesis" , "Thinker" , "Expert in established hypotheses" , "Abstract reasoner" .)