Religious teaching is unable to deny earlier beliefs (6)
I believe the answer is:
recant
'to deny earlier beliefs' is the definition.
(to recant is to reject an earlier belief)
'religious teaching is unable' is the wordplay.
'religious teaching' becomes 're' (religious education).
'is unable' becomes 'can't' (if you can't do something, you're unable to do it).
're'+'cant'='RECANT'
(Other definitions for recant that I've seen before include "Renounce (a heretical belief)" , "Renounce (a heresy)" , "Publicly withdraw" , "Formally reject a former belief" , "Withdraw former statement or belief" .)