Without sun, I’m backsliding (5)
I believe the answer is:
minus
'without' is the definition.
(minus means without)
'sun i'm backsliding' is the wordplay.
'i'm' becomes 'im' ().
'backsliding' says the letters should be written backwards.
'sun'+'im'='sunim'
'sunim' in reverse letter order is 'MINUS'.
(Other definitions for minus that I've seen before include "Deprived of" , "'Take away' sign" , "Less, without" , "deficiency" , "Negative sign - negative thing" .)