Begin study on light and reflection of the sun (5)
I believe the answer is:
solar
'of the sun' is the definition.
(solar means relating to the sun)
'begin study on light and reflection' is the wordplay.
'begin' indicates taking the first letters.
The first letters of 'study on light and reflection' is 'SOLAR'.
(Other definitions for solar that I've seen before include "Using the energy of the sun" , "revolutionary system" , "Relating to our star" , "Mediaeval chamber" , "sunny?" .)