Ray set about reading sacred text (6)
I believe the answer is:
mantra
'sacred text' is the definition.
(I know that mantra is a type of sacred text)
'ray set about reading' is the wordplay.
'ray' becomes 'manta' (manta is a kind of ray).
'set about' is an insertion indicator.
'reading' becomes 'r' (one of the traditional three R's of education).
'manta' going around 'r' is 'MANTRA'.
(Other definitions for mantra that I've seen before include "Special word repeated in Buddhist meditation" , "Vedic hymn; slogan frequently repeated" , "Hindu word or phrase as aid to meditation" , "what one keeps repeating" , "Buddhist aid to concentration" .)