Magistrate starts to regret entertaining the night before (5)
I believe the answer is:
reeve
'magistrate' is the definition.
(I've seen this before)
'starts to regret entertaining the night before' is the wordplay.
'starts to' suggests taking the first letters.
'the night before' becomes 'eve' (I've seen this before).
The first letters of 'regret entertaining' is 're'.
're'+'eve'='REEVE'
(Other definitions for reeve that I've seen before include "One of Chaucer's Canterbury pilgrims" , "Steward who told a Tale" , "Mediaeval officer; ruff's mate" , "Old magistrate or official" , "Former shire official" .)