Religious class absorbed by the Trinity (5)
I believe the answer is:
three
'trinity' is the definition.
(I know this)
'religious class absorbed by the' is the wordplay.
'religious class' becomes 'RE' (short for Religious Education).
'absorbed by' is an insertion indicator.
're' inserted within 'the' is 'THREE'.
(Other definitions for three that I've seen before include "A crowd, you might say" , "Figure" , "III" , "Trinity, triad" , "There (anag.)" .)