In the end, Clement loses the head with what he heard in confession, as it's diabolical (8)
I believe the answer is:
inhumane
'it's diabolical' is the definition.
(thesaurus)
'in the end clement loses the head with what he heard in confession' is the wordplay.
I cannot quite see how this works, but
'in' is within the answer.
'head' could be 'human' (human head is a kind of head) and 'human' is present in the answer.
'heard' could be 'e' (I've seen this in other clues) and 'e' is found in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
'as' acts as a link.
Can you help me to learn more?
(Other definitions for inhumane that I've seen before include "Not compassionate" , "Cruel" , "Callous" , "Heartless" .)