Because religious education is true (7)
I believe the answer is:
sincere
'true' is the definition.
('sincere' can be a synonym of 'true')
'because religious education' is the wordplay.
'because' becomes 'since' (I've seen this before).
'religious education' becomes 're'.
'since'+'re'='SINCERE'
'is' acts as a link.
(Other definitions for sincere that I've seen before include "Honest, frank" , "Genuine; deceit-free" , "Open and genuine, not deceitful" , "Earnest" , "Free from deception" .)