The light of day (3)
I believe the answer is:
sun
I believe this clue is a double definition.
'the light' is the first definition.
(sun is a kind of light)
'day' is the second definition.
(abbreviation for Sunday)
'of' acts as a link.
(Other definitions for sun that I've seen before include "Star around which the earth travels" , "Major star" , "Helios, in Greek myth" , "Warmer" , "Star; newspaper" .)