Biblical tribe and Biblical prophet heading off Biblical monster (9)
I believe the answer is:
leviathan
'biblical monster' is the definition.
(mythical creature mentioned in the Bible)
'biblical tribe and biblical prophet heading off' is the wordplay.
'biblical tribe' becomes 'levi' (one of the Tribes of Israel).
'and' is a charade indicator (letters next to each other).
'biblical prophet' becomes 'Nathan' (Old Testament prophet).
'heading off' means to remove the first letter.
'nathan' with its initial letter taken away is 'athan'.
'levi'+'athan'='LEVIATHAN'
(Other definitions for leviathan that I've seen before include "Monstrous beast" , "Huge (sea) monster" , "Monstrous sea creature" , "Biblical sea monster" , "Gigantic" .)