Magistrate is concerned with always finishing early (5)
I believe the answer is:
reeve
'magistrate' is the definition.
(type of historical legal official)
'concerned with always finishing early' is the wordplay.
'concerned with' becomes 're' (re can mean regarding or about).
'always' becomes 'ever' (synonyms).
'finishing early' means to remove the last letter (the word finishes earlier than normal).
'ever' with its final letter taken away is 'eve'.
're'+'eve'='REEVE'
'is' is the link.
(Other definitions for reeve that I've seen before include "See female bird" , "[PILGRIM in the CANTERBURY TALES]" , "Old-style magistrate" , "Pass a rope through a ring" , "estate supervisor" .)