Without light, I'm reflected (5)
I believe the answer is:
minus
'without' is the definition.
(minus means without)
'light i'm reflected' is the wordplay.
'light' becomes 'sun' (sun is a kind of light).
'reflected' says the letters should be written backwards (letters reflected back).
'sun'+'im'='sunim'
'sunim' back-to-front is 'MINUS'.
(Other definitions for minus that I've seen before include "Deprived of" , "Negative sign - negative thing" , "deficiency" , "Reduced by the subtraction of - thus?" , "Lacking; negative" .)