Magistrate miserable having released 50 (4)
I believe the answer is:
beak
'magistrate' is the definition.
(informal term)
'miserable having released 50' is the wordplay.
'miserable' becomes 'bleak' (bleak can mean miserable or unfortunate).
'having released' suggests deleting specific letters.
'50' becomes 'l' (Roman numeral).
'bleak' with 'l' taken out is 'BEAK'.
(Other definitions for beak that I've seen before include "Mandible" , "Bird's bill" , "Bird bill; magistrate" , "Horny mouth of a bird" , "Bird's mouth" .)