Magistrate contributes to bizarre evening (5)
I believe the answer is:
reeve
'magistrate' is the definition.
(type of historical legal official)
'contributes to bizarre evening' is the wordplay.
'contributes to' says the answer is hidden in the clue (I've seen 'contributing to' mean this (the answer contributes to part of the clue )).
'REEVE' is hidden within 'bizarre evening'.
(Other definitions for reeve that I've seen before include "One of Chaucer's Canterbury pilgrims" , "Mediaeval officer; ruff's mate" , "Old-style magistrate" , "Could 'e ever pass a rope through a hole?" , "Chief magistrate, formerly" .)