Less sun, I'm rising (5)
I believe the answer is:
minus
'less' is the definition.
(both can mean take away)
'sun i'm rising' is the wordplay.
'i'm' becomes 'im' ().
'rising' says the letters should be written backwards (going up in a down clue).
'sun'+'im'='sunim'
'sunim' written backwards gives 'MINUS'.
(Other definitions for minus that I've seen before include "deficiency" , "Take-away (sign)" , "'Take away' sign" , "Deprived of" , "(Of temperature) below zero" .)