Morning, since not a person rises (8)
I believe the answer is:
forenoon
'morning' is the definition.
(I know that morning can be written as forenoon)
'since not a person rises' is the wordplay.
'since' becomes 'for' (I've seen this in another clue).
'not a person' becomes 'no-one' (I've seen this before).
'rises' is a reversal indicator.
'noone' written backwards gives 'enoon'.
'for'+'enoon'='FORENOON'
(Other definitions for forenoon that I've seen before include "Morning (nautical)" , "twelve o'clock" , "in the morning - with 19d" .)