Lucifer turns as brown (5)
I believe the answer is:
satan
'lucifer' is the definition.
(I know that lucifer can be written as Satan)
'turns as brown' is the wordplay.
'turns' shows that the letters should be reversed in order.
'brown' becomes 'tan' (tan is a kind of brown).
'as' written backwards gives 'sa'.
'sa'+'tan'='SATAN'
(Other definitions for satan that I've seen before include "Fiend" , "Prince of Darkness" , "Diabolical figure" , "Fallen archangel" , "arch-fiend" .)