Religious education class is key once more (6)
I believe the answer is:
retype
'is key once more' is the definition.
(to type or key in again)
'religious education class' is the wordplay.
'religious education' becomes 'RE' (abbreviation).
'class' becomes 'type' (both can mean a variety or kind).
're'+'type'='RETYPE'
(Other definitions for retype that I've seen before include "impress again" , "Repeat work in pool" , "Tap out again on keyboard" , "made to revise earlier draft perhaps" , "Produce document again" .)