Bemoan unconvincing Bible books (6)
I believe the answer is:
lament
'bemoan' is the definition.
(synonyms)
'unconvincing bible books' is the wordplay.
I cannot really see how this works, but
'unconvincing' could be 'lame' (I've seen this in other clues) and 'lame' is present in the answer.
'bible' could be 'nt' (short for New Testament) and 'nt' is found in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for lament that I've seen before include "Expr. of grief; mourn" , "Expression of grief" , "Elegy" , "Express disappointment" , "Keen" .)