All the time in light and dark (3,3,5)
I believe the answer is:
day and night
'all the time' is the definition.
(I've seen this before)
'light and dark' is the wordplay.
'light' becomes 'day' (I've seen this in other clues).
'dark' becomes 'night' (it's dark at nighttime).
'day'+'and'+'night'='DAY-AND-NIGHT'
'in' is the link.
(Other definitions for day and night that I've seen before include "At all times" , "Round the clock" , "available twenty-four hours" , "24/7?" , "Rearrangement of Cole Porter's opus" .)