Biblical baddie, not half bizarre once (5)
I believe the answer is:
Here is my best explanation:
'bizarre once' is the definition.
(historical term for 'bizarre')
'biblical baddie not half' is the wordplay.
'biblical baddie' becomes 'antichrist'.
'not half' suggests halving the letters.
'antichrist' cut in half is 'ANTIC'.
(Other definitions for antic that I've seen before include "Absurd action", "Caper; grotesque (arch.)", "Playful caper", "Prank", "Absurd act", "(A) caper", "Tomfoolery", "Mad act done for fun", "Ludicrous act done for fun" and "old buffoon".)